What is the difference between a URI a URL and a URN

Question

People talk about URLs  URIs  and URNs as if they re different things  but they look the same to the naked eye   What are the distinguishable differences between them

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In order to answer this I ll lean on an answer I modified to another question  A good example of a URI is how you identify an Amazon S3 resource  Let s take   s3   www-example-com index html  fig  1   which I created as a cached copy of  http   www example com index html  fig  2   in Amazon s S3-US-West-2 datacenter   Even if StackOverflow would allow me to hyperlink to the s3    protocol scheme  it wouldn t do you any good in locating the resource  Because it Identifies a Resource  fig  1 is a valid URI  It is also a valid URN  because Amazon requires that the bucket  their term for the authority portion of the URI  be unique across datacenters  It is helpful in locating it  but it does not indicate the datacenter  Therefore it does not work as a URL   So  how do URI  URL  and URN differ in this case    fig  1 is a URI fig  1 is a URN fig  2 is a URI fig  2 is a URL The URL for fig  1 is http   www-example-com s3-website-us-west-2 amazonaws com    also http   www-example-com s3 amazonaws com index html but not http   www-example-com s3 amazonaws com   no datacenter and no filename is too generic for Amazon S3     NOTE  RFC 3986 defines URIs as scheme   authority path query fragment

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Identity   Name with Location   Every URL Uniform Resource Locator  is a URI Uniform Resource Identifier   abstractly speaking  but every URI is not a URL  There is another subcategory of URI is URN  Uniform Resource Name   which is a named resource but do not specify how to locate them  like mailto  news  ISBN is URIs   Source    URN     URN Format   urn  namespace identifier   namespace specific string     urn  and   stand for themselves  Examples    urn uuid 6e8bc430-9c3a-11d9-9669-0800200c9a66   urn ISSN 0167-6423   urn isbn 096139210x  Amazon Resource Names  ARNs  is a uniquely identify AWS resources     ARN Format   arn partition service region account-id resource     URL     URL Format    scheme     Domain  Port   path   queryString   fragmentId          and   stand for themselves  schemes are https ftp gopher mailto news telnet file man info whatis ldap      Examples      http   ip server path query ftp   ip server path mailto email-address news newsgroup-name telnet   ip server  file   ip server path segments  ldap   hostport dn attributes scope filter extensions    Analogy  To reach a person  Driving protocol others SMS  email  phone   Address hostname other phone-number  emailid  and person name object name with a relative path

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First of all get your mind out of confusion and take it simple and you will understand   URI    Uniform Resource Identifier Identifies a complete address of resource i-e location  name or both   URL    Uniform Resource Locator Identifies location of the resource   URN    Uniform Resource Name Identifies the name of the resource  Example  We have address https   www google com folder page html where   URI Uniform Resource Identifier     https   www google com folder page html  URL Uniform Resource Locator     https   www google com   URN Uniform Resource Name      folder page html  URI     URL   URN  or URL only or URN only

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Easy to explain   Lets assume the following  URI is your Name  URL is your address with your name in-order to communicate with you    my name is Loyola  Loyola is URI  my address is TN  Chennai 600001    TN  Chennai 600 001  Loyola is URL  Hope you understand    Now lets see a precise example  http   www google com fistpage html  in the above you can communicate with a page called firstpage html  URI  using following http   www google com fistpage html URL    Hence URI is subset of URL but not vice-versa

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Due to difficulties to clearly distinguish between URI and URL  as far as I remember W3C does not make a difference any longer between URI and URL  http   www w3 org Addressing

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URI is kind of the super class of URL s and URN s  Wikipedia has a fine article about them with links to the right set of RFCs

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They re the same thing  A URI is a generalization of a URL  Originally  URIs were planned to be divided into URLs  addresses  and URNs  names  but then there was little difference between a URL and URI and http URIs were used as namespaces even though they didn t actually locate any resources

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These are some very well-written but long-winded answers  Here is the difference as far as CodeIgniter is concerned   URL - http   example com some page html  URI -  some page html  Put simply  URL is the full way to indentify any resource anywhere and can have different protocols like FTP  HTTP  SCP  etc   URI is a resource on the current domain  so it needs less information to be found   In every instance that CodeIgniter uses the word URL or URI this is the difference they are talking about  though in the grand-scheme of the web  it is not 100  correct

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URI    http   en wikipedia org wiki Uniform Resource Identifier  URL s are a subset of URI s  which also contain URNs    Basically  a URI is a general identifier  where a URL specifies a location and a URN specifies a name

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Although the terms URI and URL are strictly defined  many use the terms for other things than they are defined for   Let   s take Apache for example  If http   example com foo is requested from an Apache server  you   ll have the following environment variables set    REDIRECT URL   foo REQUEST URI   foo   With mod rewrite enabled  you will also have these variables    REDIRECT SCRIPT URL   foo REDIRECT SCRIPT URI  http   example com foo SCRIPT URL   foo SCRIPT URI  http   example com foo   This might be the reason for some of the confusion

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See this document  Specifically       a URL is a type of URI that identifies a resource via a representation of its primary access mechanism  e g   its network  location    rather than by some other attributes it may have    It s not an extremely clear term  really

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URI  URL  URN   As the image above indicates  there are three distinct components at play here  It   s usually best to go to the source when discussing matters like these  so here   s an exerpt from Tim Berners-Lee  et  al  in   RFC 3986  Uniform Resource Identifier  URI   Generic Syntax      A Uniform Resource Identifier  URI  is a compact sequence of   characters that identifies an abstract or physical resource       A URI can be further classified as a locator  a name  or both  The   term    Uniform Resource Locator     URL  refers to the subset of URIs   that  in addition to identifying a resource  provide a means of   locating the resource by describing its primary access mechanism    e g   its network    location

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I was wondering about the same thing and I ve found this  http   docs kohanaphp com helpers url   You can see a clear example using the url  current   method  If you have this URL  http   example com kohana index php welcome home html query string then using url current   gives you the URI which  according to the documentation  is  welcome home

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URI is kind of the super class of URL s and URN s  Wikipedia has a fine article about them with links to the right set of RFCs

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The best  technical  summary imo is this one  IRI  URI  URL  URN and their differences from Jan Martin Keil   IRI  URI  URL  URN and their differences  Everybody dealing with the Semantic Web repeatedly comes across the terms IRI  URI  URL and URN  Nevertheless  I frequently observe that there is some confusion about their exact meaning  And  of course  others noticed that as well  see e g  RFC3305 or search on Google   To be honest  I even was confused myself at the outset  But actually the issue is not that complex  Let   s have a look on the definitions of the mentioned terms to see what the differences are   URI  A Uniform Resource Identifier is a compact sequence of characters that identifies an abstract or physical resource  The set of characters is limited to US-ASCII excluding some reserved characters  Characters outside the set of allowed characters can be represented using Percent-Encoding  A URI can be used as a locator  a name  or both  If a URI is a locator  it describes a resource   s primary access mechanism  If a URI is a name  it identifies a resource by giving it a unique name  The exact specifications of syntax and semantics of a URI depend on the used Scheme that is defined by the characters before the first colon   RFC3986   URN  A Uniform Resource Name is a URI in the scheme urn intended to serve as persistent  location-independent  resource identifier  Historically  the term also referred to any URI   RFC3986  A URN consists of a Namespace Identifier  NID  and a Namespace Specific String  NSS   urn   The syntax and semantics of the NSS is specific specific for each NID  Beside the registered NIDs  there exist several more NIDs  that did not go through the official registration process   RFC2141   URL  A Uniform Resource Locator is a URI that  in addition to identifying a resource  provides a means of locating the resource by describing its primary access mechanism  RFC3986   As there is no exact definition of URL by means of a set of Schemes   URL is a useful but informal concept   usually referring to a subset of URIs that do not contain URNs  RFC3305    IRI  An Internationalized Resource Identifier is defined similarly to a URI  but the character set is extended to the Universal Coded Character Set  Therefore  it can contain any Latin and non Latin characters except the reserved characters  Instead of extending the definition of URI  the term IRI was introduced to allow for a clear distinction and avoid incompatibilities  IRIs are meant to replace URIs in identifying resources in situations where the Universal Coded Character Set is supported  By definition  every URI is an IRI  Furthermore  there is a defined surjective mapping of IRIs to URIs  Every IRI can be mapped to exactly one URI  but different IRIs might map to the same URI  Therefore  the conversion back from a URI to an IRI may not produce the original IRI   RFC3987   Summarizing we can say   IRI is a superset of URI  IRI   URI  URI is a superset of URL  URI   URL  URI is a superset of URN  URI   URN  URL and URN are disjoint  URL n URN         Conclusions for Semantic Web Issues  RDF explicitly allows to use IRIs to name entities  RFC3987   This means that we can use almost every character in entity names  On the other hand  we often have to deal with early state software  Thus  it is not unlikely to run into problems using non ASCII characters  Therefore  I suggest to avoid non URI names for entities and recommend to use http URIs  LINKED-DATA   To put it briefly  only use URLs to name your entities  Of course  we can refer to existing entities named by a URN  However  we should avoid to newly create this kind of identifiers

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URI    http   en wikipedia org wiki Uniform Resource Identifier  URL s are a subset of URI s  which also contain URNs    Basically  a URI is a general identifier  where a URL specifies a location and a URN specifies a name

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First of all get your mind out of confusion and take it simple and you will understand   URI    Uniform Resource Identifier Identifies a complete address of resource i-e location  name or both   URL    Uniform Resource Locator Identifies location of the resource   URN    Uniform Resource Name Identifies the name of the resource  Example  We have address https   www google com folder page html where   URI Uniform Resource Identifier     https   www google com folder page html  URL Uniform Resource Locator     https   www google com   URN Uniform Resource Name      folder page html  URI     URL   URN  or URL only or URN only

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As per RFC 3986  URIs are comprised of the following pieces   scheme   authority path query   The URI describes the protocol for accessing a resource  path  or application  query  on a server  authority      All the URLs are URIs  and all the URNs are URIs  but all the URIs are not URLs   Please refer for more details      Wikipedia

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URIs came about from the need to identify resources on the Web  and other Internet resources such as electronic mailboxes in a uniform and coherent way  So  one can introduce a new type of widget  URIs to identify widget resources or use tel  URIs to have web links cause telephone calls to be made when invoked     Some URIs provide information to locate a resource  such as a DNS host name and a path on that machine   while some are used as pure resource names  The URL is reserved for identifiers that are resource locators  including  http  URLs such as http   stackoverflow com  which identifies the web page at the given path on the host  Another example is  mailto  URLs  such as mailto fred mail org  which identifies the mailbox at the given address   URNs are URIs that are used as pure resource names rather than locators  For example  the URI  mid 0E4FC272-5C02-11D9-B115-000A95B55BC8 stackoverflow com is a URN that identifies the email message containing it in its  Message-Id  field  The URI serves to distinguish that message from any other email message  But it does not itself provide the message s address in any store

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URI    http   en wikipedia org wiki Uniform Resource Identifier  URL s are a subset of URI s  which also contain URNs    Basically  a URI is a general identifier  where a URL specifies a location and a URN specifies a name

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Don t forget URNs  URIs and URLs are both URNs   URLs have a location   URI  foo URL  http   some domain com foo URL  http   some domain com 8080 foo URL  ftp   some domain com foo   They re all URNs

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Easy to explain   Lets assume the following  URI is your Name  URL is your address with your name in-order to communicate with you    my name is Loyola  Loyola is URI  my address is TN  Chennai 600001    TN  Chennai 600 001  Loyola is URL  Hope you understand    Now lets see a precise example  http   www google com fistpage html  in the above you can communicate with a page called firstpage html  URI  using following http   www google com fistpage html URL    Hence URI is subset of URL but not vice-versa

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After reading through the posts  I find some very relevant comments  In short  the confusion between the URL and URI definitions is based in part on which definition depends on which and also informal use of the word URI in software development   By definition URL is a subset of URI  RFC2396   URI contain URN and URL  Both URI and URL each have their own specific syntax that confers upon them the status of being either URI or URL   URN are for uniquely identifying a resource while URL are for locating a resource  Note that a resource can have more than one URL but only a single URN  RFC2611   As web developers and programmers we will almost always be concerned with URL and therefore URI  Now a URL is specifically defined to have all the parts  scheme scheme-specific-part  like for example https   stackoverflow com questions  This is a URL and it is also a URI  Now consider a relative link embedded in the page such as    index html  This is no longer a URL by definition  It is still what is referred to as a  URI-reference   RFC2396     I believe that when the word URI is used to refer to relative paths   URI-reference  is actually what is being thought of  So informally  software systems use URI to refer to relative pathing and URL for the absolute address  So in this sense  a relative path is no longer a URL but still URI

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Although the terms URI and URL are strictly defined  many use the terms for other things than they are defined for   Let   s take Apache for example  If http   example com foo is requested from an Apache server  you   ll have the following environment variables set    REDIRECT URL   foo REQUEST URI   foo   With mod rewrite enabled  you will also have these variables    REDIRECT SCRIPT URL   foo REDIRECT SCRIPT URI  http   example com foo SCRIPT URL   foo SCRIPT URI  http   example com foo   This might be the reason for some of the confusion

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From RFC 3986   A URI can be further classified as a locator  a name  or both   The term  quot Uniform Resource Locator quot   URL  refers to the subset of URIs that  in addition to identifying a resource  provide a means of locating the resource by describing its primary access mechanism  e g   its network  quot location quot     The term  quot Uniform Resource Name quot   URN  has been used historically to refer to both URIs under the  quot urn quot  scheme  RFC2141   which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable  and to any other URI with the properties of a name   So all URLs are URIs  and all URNs are URIs - but URNs and URLs are different  so you can t say that all URIs are URLs  If you haven t already read Roger Pate s answer  I d advise doing so as well

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Due to difficulties to clearly distinguish between URI and URL  as far as I remember W3C does not make a difference any longer between URI and URL  http   www w3 org Addressing

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URI -- Uniform Resource Identifier  URIs are a standard for identifying documents using a short string of numbers  letters  and symbols    They are defined by RFC 3986 - Uniform Resource Identifier  URI   Generic Syntax   URLs  URNs  and URCs are all types of URI     URL -- Uniform Resource Locator  Contains information about how to fetch a resource from its location   For example    http   example com mypage html ftp   example com download zip mailto user example com file    home user file txt tel 1-888-555-5555 http   example com resource foo bar fragment  other link html  A relative URL  only useful in the context of another URL    URLs always start with a protocol  http  and usually contain information such as the network host name  example com  and often a document path   foo mypage html   URLs may have query parameters and fragment identifiers   URN -- Uniform Resource Name  Identifies a resource by a unique and persistent name  but doesn t necessarily tell you how to locate it on the internet    It usually starts with the prefix urn   For example    urn isbn 0451450523 to identify a book by its ISBN number  urn uuid 6e8bc430-9c3a-11d9-9669-0800200c9a66 a globally unique identifier urn publishing book - An XML namespace that identifies the document as a type of book    URNs can identify ideas and concepts   They are not restricted to identifying documents    When a URN does represent a document  it can be translated into a URL by a  resolver    The document can then be downloaded from the URL   URC -- Uniform Resource Citation  Points to meta data about a document rather than to the document itself    An example of a URC is one that points to the HTML source code of a page like  view-source http   example com   Data URI  Rather than locating it on the internet  or naming it  data can be placed directly into a URI   An example would be data  Hello 20World       Frequently Asked Questions  I ve heard that I shouldn t say URL anymore  why   The W3 spec for HTML says that the href of an anchor tag can contain a URI  not just a URL   You should be able to put in a URN such as  lt a href  urn isbn 0451450523  gt    Your browser would then resolve that URN to a URL and download the book for you   Do any browsers actually know how to fetch documents by URN   Not that I know of  but modern web browser do implement the data URI scheme   Does the difference between URL and URI have anything to do with whether it is relative or absolute   No    Both relative and absolute URLs are URLs  and URIs    Does the difference between URL and URI have anything to do with whether it has query parameters   No    Both URLs with and without query parameters are URLs  and URIs    Does the difference between URL and URI have anything to do with whether it has a fragment identifier   No    Both URLs with and without fragment identifiers are URLs  and URIs    Does the difference between URL and URI have anything to do with what characters are permitted   No  URLs are defined to be a strict subset of URIs    If a parser allows a character in a URL but not in a URI  there is a bug in the parser    The specs go into great detail about which characters are allowed in which parts of URLs and URIs    Some characters may be allowed only in some parts of the URL  but characters alone are not a difference between URLs and URIs   But doesn t the W3C now say that URLs and URIs are the same thing   Yes   The W3C realized that there is a ton of confusion about this   They issued a URI clarification document that says that it is now OK to use the terms URL and URI interchangeably  to mean URI    It is no longer useful to strictly segment URIs into different types such as URL  URN  and URC   Can a URI be both a URL and a URN   The definition of URN is now looser than what I stated above   The latest RFC on URIs says that any URI can now be a URN  regardless of whether it starts with urn   as long as it has  the properties of a name    That is  It is globally unique and persistent even when the resource ceases to exist or becomes unavailable   An example  The URIs used in HTML doctypes such as http   www w3 org TR html4 strict dtd    That URI would continue to name the HTML4 transitional doctype even if the page on the w3 org website were deleted

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URL  A URL is a specialization of URI that defines the network location of a specific resource  Unlike a URN  the URL defines how the resource can be obtained  We use URLs every day in the form of http   example com etc  But a URL doesn t have to be an HTTP URL  it can be ftp   example com etc   too   URI  A URI identifies a resource either by location  or a name  or both  More often than not  most of us use URIs that defines a location to a resource  The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opinion  A URI has two specializations known as URL and URN   Difference between URL and URI  A URI is an identifier for some resource  but a URL gives you specific information as to obtain that resource  A URI is a URL and as one commenter pointed out  it is now considered incorrect to use URL when describing applications  Generally  if the URL describes both the location and name of a resource  the term to use is URI  Since this is generally the case most of us encounter everyday  URI is the correct term

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A small addition to the answers already posted  here s a Venn s diagram to sum up the theory  from Prateek Joshi s beautiful explanation      And an example  also from Prateek s website

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From RFC 3986   A URI can be further classified as a locator  a name  or both   The term  quot Uniform Resource Locator quot   URL  refers to the subset of URIs that  in addition to identifying a resource  provide a means of locating the resource by describing its primary access mechanism  e g   its network  quot location quot     The term  quot Uniform Resource Name quot   URN  has been used historically to refer to both URIs under the  quot urn quot  scheme  RFC2141   which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable  and to any other URI with the properties of a name   So all URLs are URIs  and all URNs are URIs - but URNs and URLs are different  so you can t say that all URIs are URLs  If you haven t already read Roger Pate s answer  I d advise doing so as well

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They re the same thing  A URI is a generalization of a URL  Originally  URIs were planned to be divided into URLs  addresses  and URNs  names  but then there was little difference between a URL and URI and http URIs were used as namespaces even though they didn t actually locate any resources

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URIs identify and URLs locate  however  locators are also identifiers  so every URL is also a URI  but there are URIs which are not URLs  Examples  Roger Pate  This is my name  which is an identifier  It is like a URI  but cannot be a URL  as it tells you nothing about my location or how to contact me  In this case it also happens to identify at least 5 other people in the USA alone   4914 West Bay Street  Nassau  Bahamas  This is a locator  which is an identifier for that physical location  It is like both a URL and URI  since all URLs are URIs   and also identifies me indirectly as  quot resident of   quot   In this case it uniquely identifies me  but that would change if I get a roommate  I say  quot like quot  because these examples do not follow the required syntax  Popular confusion From Wikipedia   In computing  a Uniform Resource Locator  URL  is a subset of the Uniform Resource Identifier  URI  that specifies where an identified resource is available and the mechanism for retrieving it  In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI       emphasis mine   Because of this common confusion  many products and documentation incorrectly use one term instead of the other  assign their own distinction  or use them synonymously  URNs My name  Roger Pate  could be like a URN  Uniform Resource Name   except those are much more regulated and intended to be unique across both space and time  Because I currently share this name with other people  it s not globally unique and would not be appropriate as a URN   However  even if no other family used this name  I m named after my paternal grandfather  so it still wouldn t be unique across time   And even if that wasn t the case  the possibility of naming my descendants after me make this unsuitable as a URN  URNs are different from URLs in this rigid uniqueness constraint  even though they both share the syntax of URIs

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In summary  a URI identifies  a URL identifies and locates   Consider a specific edition of Shakespeare s play Romeo and Juliet  of which you have a digital copy on your home network   You could identify the text as urn isbn 0-486-27557-4  That would be a URI  but more specifically a URN  because it names the text   You could also identify the text as file   hostname sharename RomeoAndJuliet pdf  That would also be a URI  but more specifically a URL because it locates the text    Uniform Resource Name   Note that my example is adapted from Wikipedia

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I found     A uniform resource identifier URI  represents something of a big picture  You can split URIs  URIs can be classified as locators  uniform resource locators- URL   or as names  uniform resource name-URN   or either both  So basically  a URN functions like a person s name and the URL depicts that person s address  So long story short  a URN defines an item s identity  while the URL provides defines the method for finding it  finally encapsulating these two concepts is the URI

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As per RFC 3986  URIs are comprised of the following pieces   scheme   authority path query   The URI describes the protocol for accessing a resource  path  or application  query  on a server  authority      All the URLs are URIs  and all the URNs are URIs  but all the URIs are not URLs   Please refer for more details      Wikipedia

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These are some very well-written but long-winded answers  Here is the difference as far as CodeIgniter is concerned   URL - http   example com some page html  URI -  some page html  Put simply  URL is the full way to indentify any resource anywhere and can have different protocols like FTP  HTTP  SCP  etc   URI is a resource on the current domain  so it needs less information to be found   In every instance that CodeIgniter uses the word URL or URI this is the difference they are talking about  though in the grand-scheme of the web  it is not 100  correct

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This is one of the most confusing and possibly irrelevant topics I ve encountered as a web professional  As I understand it  a URI is a description of something  following an accepted format  that can define both or either the unique name  identification  of something or its location  There are two basic subsets   URLs  which define location  especially to a browser trying to look up a webpage  and URNs  which define the unique name of something   I tend to think of URNs as being similar to GUIDs   They are simply a standardized methodology for providing unique names for things   As in the namespace declarative that uses a company s name   it s not like there is a resource sitting on a server somewhere to correspond to that line of text   it simply uniquely identifies something  I also tend to completely avoid the term URI and discuss things only in terms of URL or URN as appropriate  because it causes so much confusion   The question we should really try answering for people isn t so much the semantics  but how to identify when encountering the terms whether or not there is any practical difference in them that will change the approach to a programming situation   For example  if someone corrects me in conversation and says   quot oh  that s not a URL it s a URI quot  I know they re full of it   If someone says   quot we re using a URN to define the resource  quot  I m more likely to understand we are only naming it uniquely  not locating it on a server  If I m way off base  please let me know

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They re the same thing  A URI is a generalization of a URL  Originally  URIs were planned to be divided into URLs  addresses  and URNs  names  but then there was little difference between a URL and URI and http URIs were used as namespaces even though they didn t actually locate any resources

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URI is kind of the super class of URL s and URN s  Wikipedia has a fine article about them with links to the right set of RFCs

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Don t forget URNs  URIs and URLs are both URNs   URLs have a location   URI  foo URL  http   some domain com foo URL  http   some domain com 8080 foo URL  ftp   some domain com foo   They re all URNs

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Here is my simplification   URN  unique resource name  i e   what   eg urn issn 1234-5678    This is meant to be unique    as in no two different docs can have the same urn  A bit like  uuid   URL   where  to find it   eg https   google com pub issnid 1234-5678    or                               ftp   somesite com doc8 pdf    URI  can be either a URN or a URL  This fuzzy definition is thanks to RFC 3986 produced by W3C and IETF   The definition of URI has changed over the years  so it makes sense for most people to be confused  However  you can now take solace in the fact that you can refer to http   somesite com something as either a URL or URI      an you will be right either way  at least fot the time being anyway

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The best  technical  summary imo is this one  IRI  URI  URL  URN and their differences from Jan Martin Keil   IRI  URI  URL  URN and their differences  Everybody dealing with the Semantic Web repeatedly comes across the terms IRI  URI  URL and URN  Nevertheless  I frequently observe that there is some confusion about their exact meaning  And  of course  others noticed that as well  see e g  RFC3305 or search on Google   To be honest  I even was confused myself at the outset  But actually the issue is not that complex  Let   s have a look on the definitions of the mentioned terms to see what the differences are   URI  A Uniform Resource Identifier is a compact sequence of characters that identifies an abstract or physical resource  The set of characters is limited to US-ASCII excluding some reserved characters  Characters outside the set of allowed characters can be represented using Percent-Encoding  A URI can be used as a locator  a name  or both  If a URI is a locator  it describes a resource   s primary access mechanism  If a URI is a name  it identifies a resource by giving it a unique name  The exact specifications of syntax and semantics of a URI depend on the used Scheme that is defined by the characters before the first colon   RFC3986   URN  A Uniform Resource Name is a URI in the scheme urn intended to serve as persistent  location-independent  resource identifier  Historically  the term also referred to any URI   RFC3986  A URN consists of a Namespace Identifier  NID  and a Namespace Specific String  NSS   urn   The syntax and semantics of the NSS is specific specific for each NID  Beside the registered NIDs  there exist several more NIDs  that did not go through the official registration process   RFC2141   URL  A Uniform Resource Locator is a URI that  in addition to identifying a resource  provides a means of locating the resource by describing its primary access mechanism  RFC3986   As there is no exact definition of URL by means of a set of Schemes   URL is a useful but informal concept   usually referring to a subset of URIs that do not contain URNs  RFC3305    IRI  An Internationalized Resource Identifier is defined similarly to a URI  but the character set is extended to the Universal Coded Character Set  Therefore  it can contain any Latin and non Latin characters except the reserved characters  Instead of extending the definition of URI  the term IRI was introduced to allow for a clear distinction and avoid incompatibilities  IRIs are meant to replace URIs in identifying resources in situations where the Universal Coded Character Set is supported  By definition  every URI is an IRI  Furthermore  there is a defined surjective mapping of IRIs to URIs  Every IRI can be mapped to exactly one URI  but different IRIs might map to the same URI  Therefore  the conversion back from a URI to an IRI may not produce the original IRI   RFC3987   Summarizing we can say   IRI is a superset of URI  IRI   URI  URI is a superset of URL  URI   URL  URI is a superset of URN  URI   URN  URL and URN are disjoint  URL n URN         Conclusions for Semantic Web Issues  RDF explicitly allows to use IRIs to name entities  RFC3987   This means that we can use almost every character in entity names  On the other hand  we often have to deal with early state software  Thus  it is not unlikely to run into problems using non ASCII characters  Therefore  I suggest to avoid non URI names for entities and recommend to use http URIs  LINKED-DATA   To put it briefly  only use URLs to name your entities  Of course  we can refer to existing entities named by a URN  However  we should avoid to newly create this kind of identifiers

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Here is my simplification   URN  unique resource name  i e   what   eg urn issn 1234-5678    This is meant to be unique    as in no two different docs can have the same urn  A bit like  uuid   URL   where  to find it   eg https   google com pub issnid 1234-5678    or                               ftp   somesite com doc8 pdf    URI  can be either a URN or a URL  This fuzzy definition is thanks to RFC 3986 produced by W3C and IETF   The definition of URI has changed over the years  so it makes sense for most people to be confused  However  you can now take solace in the fact that you can refer to http   somesite com something as either a URL or URI      an you will be right either way  at least fot the time being anyway

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Wikipedia will give all the information you need here  Quoting from http   en wikipedia org wiki URI      A URL is a URI that  in addition to identifying a resource  provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network  location

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A small addition to the answers already posted  here s a Venn s diagram to sum up the theory  from Prateek Joshi s beautiful explanation      And an example  also from Prateek s website

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The answer is ambiguous  In Java it is frequently used in this way   An Uniform Resource Locator  URL  is the term used to identify an Internet resource including the scheme  http  https  ftp  news  etc    For instance What is the difference between a URI  a URL and a URN   An Uniform Resource Identifier  URI  is used to identify a single document in the Web Server  For instance  questions 176264 whats-the-difference-between-a-uri-and-a-url  In Java servlets  the URI frequently refers to the document without the web application context

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From RFC 3986   A URI can be further classified as a locator  a name  or both   The term  quot Uniform Resource Locator quot   URL  refers to the subset of URIs that  in addition to identifying a resource  provide a means of locating the resource by describing its primary access mechanism  e g   its network  quot location quot     The term  quot Uniform Resource Name quot   URN  has been used historically to refer to both URIs under the  quot urn quot  scheme  RFC2141   which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable  and to any other URI with the properties of a name   So all URLs are URIs  and all URNs are URIs - but URNs and URLs are different  so you can t say that all URIs are URLs  If you haven t already read Roger Pate s answer  I d advise doing so as well

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URI  URL  URN   As the image above indicates  there are three distinct components at play here  It   s usually best to go to the source when discussing matters like these  so here   s an exerpt from Tim Berners-Lee  et  al  in   RFC 3986  Uniform Resource Identifier  URI   Generic Syntax      A Uniform Resource Identifier  URI  is a compact sequence of   characters that identifies an abstract or physical resource       A URI can be further classified as a locator  a name  or both  The   term    Uniform Resource Locator     URL  refers to the subset of URIs   that  in addition to identifying a resource  provide a means of   locating the resource by describing its primary access mechanism    e g   its network    location

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Identity   Name with Location   Every URL Uniform Resource Locator  is a URI Uniform Resource Identifier   abstractly speaking  but every URI is not a URL  There is another subcategory of URI is URN  Uniform Resource Name   which is a named resource but do not specify how to locate them  like mailto  news  ISBN is URIs   Source    URN     URN Format   urn  namespace identifier   namespace specific string     urn  and   stand for themselves  Examples    urn uuid 6e8bc430-9c3a-11d9-9669-0800200c9a66   urn ISSN 0167-6423   urn isbn 096139210x  Amazon Resource Names  ARNs  is a uniquely identify AWS resources     ARN Format   arn partition service region account-id resource     URL     URL Format    scheme     Domain  Port   path   queryString   fragmentId          and   stand for themselves  schemes are https ftp gopher mailto news telnet file man info whatis ldap      Examples      http   ip server path query ftp   ip server path mailto email-address news newsgroup-name telnet   ip server  file   ip server path segments  ldap   hostport dn attributes scope filter extensions    Analogy  To reach a person  Driving protocol others SMS  email  phone   Address hostname other phone-number  emailid  and person name object name with a relative path

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A URI identifies a resource either by location  or a name  or both  More often than not  most of us use URIs that defines a location to a resource  The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opinion  A URI has two specializations known as URL and URN   A URL is a specialization of URI that defines the network location of a specific resource  Unlike a URN  the URL defines how the resource can be obtained  We use URLs every day in the form of http   stackoverflow com  etc  But a URL doesn   t have to be an HTTP URL  it can be ftp   example com  etc

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From RFC 3986   A URI can be further classified as a locator  a name  or both   The term  quot Uniform Resource Locator quot   URL  refers to the subset of URIs that  in addition to identifying a resource  provide a means of locating the resource by describing its primary access mechanism  e g   its network  quot location quot     The term  quot Uniform Resource Name quot   URN  has been used historically to refer to both URIs under the  quot urn quot  scheme  RFC2141   which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable  and to any other URI with the properties of a name   So all URLs are URIs  and all URNs are URIs - but URNs and URLs are different  so you can t say that all URIs are URLs  If you haven t already read Roger Pate s answer  I d advise doing so as well

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After reading through the posts  I find some very relevant comments  In short  the confusion between the URL and URI definitions is based in part on which definition depends on which and also informal use of the word URI in software development   By definition URL is a subset of URI  RFC2396   URI contain URN and URL  Both URI and URL each have their own specific syntax that confers upon them the status of being either URI or URL   URN are for uniquely identifying a resource while URL are for locating a resource  Note that a resource can have more than one URL but only a single URN  RFC2611   As web developers and programmers we will almost always be concerned with URL and therefore URI  Now a URL is specifically defined to have all the parts  scheme scheme-specific-part  like for example https   stackoverflow com questions  This is a URL and it is also a URI  Now consider a relative link embedded in the page such as    index html  This is no longer a URL by definition  It is still what is referred to as a  URI-reference   RFC2396     I believe that when the word URI is used to refer to relative paths   URI-reference  is actually what is being thought of  So informally  software systems use URI to refer to relative pathing and URL for the absolute address  So in this sense  a relative path is no longer a URL but still URI

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URI    http   en wikipedia org wiki Uniform Resource Identifier  URL s are a subset of URI s  which also contain URNs    Basically  a URI is a general identifier  where a URL specifies a location and a URN specifies a name

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I was wondering about the same thing and I ve found this  http   docs kohanaphp com helpers url   You can see a clear example using the url  current   method  If you have this URL  http   example com kohana index php welcome home html query string then using url current   gives you the URI which  according to the documentation  is  welcome home

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URI -- Uniform Resource Identifier  URIs are a standard for identifying documents using a short string of numbers  letters  and symbols    They are defined by RFC 3986 - Uniform Resource Identifier  URI   Generic Syntax   URLs  URNs  and URCs are all types of URI     URL -- Uniform Resource Locator  Contains information about how to fetch a resource from its location   For example    http   example com mypage html ftp   example com download zip mailto user example com file    home user file txt tel 1-888-555-5555 http   example com resource foo bar fragment  other link html  A relative URL  only useful in the context of another URL    URLs always start with a protocol  http  and usually contain information such as the network host name  example com  and often a document path   foo mypage html   URLs may have query parameters and fragment identifiers   URN -- Uniform Resource Name  Identifies a resource by a unique and persistent name  but doesn t necessarily tell you how to locate it on the internet    It usually starts with the prefix urn   For example    urn isbn 0451450523 to identify a book by its ISBN number  urn uuid 6e8bc430-9c3a-11d9-9669-0800200c9a66 a globally unique identifier urn publishing book - An XML namespace that identifies the document as a type of book    URNs can identify ideas and concepts   They are not restricted to identifying documents    When a URN does represent a document  it can be translated into a URL by a  resolver    The document can then be downloaded from the URL   URC -- Uniform Resource Citation  Points to meta data about a document rather than to the document itself    An example of a URC is one that points to the HTML source code of a page like  view-source http   example com   Data URI  Rather than locating it on the internet  or naming it  data can be placed directly into a URI   An example would be data  Hello 20World       Frequently Asked Questions  I ve heard that I shouldn t say URL anymore  why   The W3 spec for HTML says that the href of an anchor tag can contain a URI  not just a URL   You should be able to put in a URN such as  lt a href  urn isbn 0451450523  gt    Your browser would then resolve that URN to a URL and download the book for you   Do any browsers actually know how to fetch documents by URN   Not that I know of  but modern web browser do implement the data URI scheme   Does the difference between URL and URI have anything to do with whether it is relative or absolute   No    Both relative and absolute URLs are URLs  and URIs    Does the difference between URL and URI have anything to do with whether it has query parameters   No    Both URLs with and without query parameters are URLs  and URIs    Does the difference between URL and URI have anything to do with whether it has a fragment identifier   No    Both URLs with and without fragment identifiers are URLs  and URIs    Does the difference between URL and URI have anything to do with what characters are permitted   No  URLs are defined to be a strict subset of URIs    If a parser allows a character in a URL but not in a URI  there is a bug in the parser    The specs go into great detail about which characters are allowed in which parts of URLs and URIs    Some characters may be allowed only in some parts of the URL  but characters alone are not a difference between URLs and URIs   But doesn t the W3C now say that URLs and URIs are the same thing   Yes   The W3C realized that there is a ton of confusion about this   They issued a URI clarification document that says that it is now OK to use the terms URL and URI interchangeably  to mean URI    It is no longer useful to strictly segment URIs into different types such as URL  URN  and URC   Can a URI be both a URL and a URN   The definition of URN is now looser than what I stated above   The latest RFC on URIs says that any URI can now be a URN  regardless of whether it starts with urn   as long as it has  the properties of a name    That is  It is globally unique and persistent even when the resource ceases to exist or becomes unavailable   An example  The URIs used in HTML doctypes such as http   www w3 org TR html4 strict dtd    That URI would continue to name the HTML4 transitional doctype even if the page on the w3 org website were deleted

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The answer is ambiguous  In Java it is frequently used in this way   An Uniform Resource Locator  URL  is the term used to identify an Internet resource including the scheme  http  https  ftp  news  etc    For instance What is the difference between a URI  a URL and a URN   An Uniform Resource Identifier  URI  is used to identify a single document in the Web Server  For instance  questions 176264 whats-the-difference-between-a-uri-and-a-url  In Java servlets  the URI frequently refers to the document without the web application context

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See this document  Specifically       a URL is a type of URI that identifies a resource via a representation of its primary access mechanism  e g   its network  location    rather than by some other attributes it may have    It s not an extremely clear term  really

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In summary  a URI identifies  a URL identifies and locates   Consider a specific edition of Shakespeare s play Romeo and Juliet  of which you have a digital copy on your home network   You could identify the text as urn isbn 0-486-27557-4  That would be a URI  but more specifically a URN  because it names the text   You could also identify the text as file   hostname sharename RomeoAndJuliet pdf  That would also be a URI  but more specifically a URL because it locates the text    Uniform Resource Name   Note that my example is adapted from Wikipedia

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URL  A URL is a specialization of URI that defines the network location of a specific resource  Unlike a URN  the URL defines how the resource can be obtained  We use URLs every day in the form of http   example com etc  But a URL doesn t have to be an HTTP URL  it can be ftp   example com etc   too   URI  A URI identifies a resource either by location  or a name  or both  More often than not  most of us use URIs that defines a location to a resource  The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opinion  A URI has two specializations known as URL and URN   Difference between URL and URI  A URI is an identifier for some resource  but a URL gives you specific information as to obtain that resource  A URI is a URL and as one commenter pointed out  it is now considered incorrect to use URL when describing applications  Generally  if the URL describes both the location and name of a resource  the term to use is URI  Since this is generally the case most of us encounter everyday  URI is the correct term

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Wikipedia will give all the information you need here  Quoting from http   en wikipedia org wiki URI      A URL is a URI that  in addition to identifying a resource  provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network  location

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URIs identify and URLs locate  however  locators are also identifiers  so every URL is also a URI  but there are URIs which are not URLs  Examples  Roger Pate  This is my name  which is an identifier  It is like a URI  but cannot be a URL  as it tells you nothing about my location or how to contact me  In this case it also happens to identify at least 5 other people in the USA alone   4914 West Bay Street  Nassau  Bahamas  This is a locator  which is an identifier for that physical location  It is like both a URL and URI  since all URLs are URIs   and also identifies me indirectly as  quot resident of   quot   In this case it uniquely identifies me  but that would change if I get a roommate  I say  quot like quot  because these examples do not follow the required syntax  Popular confusion From Wikipedia   In computing  a Uniform Resource Locator  URL  is a subset of the Uniform Resource Identifier  URI  that specifies where an identified resource is available and the mechanism for retrieving it  In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI       emphasis mine   Because of this common confusion  many products and documentation incorrectly use one term instead of the other  assign their own distinction  or use them synonymously  URNs My name  Roger Pate  could be like a URN  Uniform Resource Name   except those are much more regulated and intended to be unique across both space and time  Because I currently share this name with other people  it s not globally unique and would not be appropriate as a URN   However  even if no other family used this name  I m named after my paternal grandfather  so it still wouldn t be unique across time   And even if that wasn t the case  the possibility of naming my descendants after me make this unsuitable as a URN  URNs are different from URLs in this rigid uniqueness constraint  even though they both share the syntax of URIs

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URIs came about from the need to identify resources on the Web  and other Internet resources such as electronic mailboxes in a uniform and coherent way  So  one can introduce a new type of widget  URIs to identify widget resources or use tel  URIs to have web links cause telephone calls to be made when invoked     Some URIs provide information to locate a resource  such as a DNS host name and a path on that machine   while some are used as pure resource names  The URL is reserved for identifiers that are resource locators  including  http  URLs such as http   stackoverflow com  which identifies the web page at the given path on the host  Another example is  mailto  URLs  such as mailto fred mail org  which identifies the mailbox at the given address   URNs are URIs that are used as pure resource names rather than locators  For example  the URI  mid 0E4FC272-5C02-11D9-B115-000A95B55BC8 stackoverflow com is a URN that identifies the email message containing it in its  Message-Id  field  The URI serves to distinguish that message from any other email message  But it does not itself provide the message s address in any store

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They re the same thing  A URI is a generalization of a URL  Originally  URIs were planned to be divided into URLs  addresses  and URNs  names  but then there was little difference between a URL and URI and http URIs were used as namespaces even though they didn t actually locate any resources

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Wikipedia will give all the information you need here  Quoting from http   en wikipedia org wiki URI      A URL is a URI that  in addition to identifying a resource  provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network  location

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Another example I like to use when thinking about URIs is the xmlns attribute of an XML document    lt rootElement xmlns myPrefix  com mycompany mynode  gt       lt myPrefix aNode gt some text lt  myPrefix aNode gt   lt  rootElement gt    In this case com mycompany mynode would be a URI that uniquely identifies the  myPrefix  namespace for all of the elements that use it within my XML document  This is NOT a URL because it is only used to identify  not to locate something per se

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A URI identifies a resource either by location  or a name  or both  More often than not  most of us use URIs that defines a location to a resource  The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opinion  A URI has two specializations known as URL and URN   A URL is a specialization of URI that defines the network location of a specific resource  Unlike a URN  the URL defines how the resource can be obtained  We use URLs every day in the form of http   stackoverflow com  etc  But a URL doesn   t have to be an HTTP URL  it can be ftp   example com  etc

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Another example I like to use when thinking about URIs is the xmlns attribute of an XML document    lt rootElement xmlns myPrefix  com mycompany mynode  gt       lt myPrefix aNode gt some text lt  myPrefix aNode gt   lt  rootElement gt    In this case com mycompany mynode would be a URI that uniquely identifies the  myPrefix  namespace for all of the elements that use it within my XML document  This is NOT a URL because it is only used to identify  not to locate something per se

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In order to answer this I ll lean on an answer I modified to another question  A good example of a URI is how you identify an Amazon S3 resource  Let s take   s3   www-example-com index html  fig  1   which I created as a cached copy of  http   www example com index html  fig  2   in Amazon s S3-US-West-2 datacenter   Even if StackOverflow would allow me to hyperlink to the s3    protocol scheme  it wouldn t do you any good in locating the resource  Because it Identifies a Resource  fig  1 is a valid URI  It is also a valid URN  because Amazon requires that the bucket  their term for the authority portion of the URI  be unique across datacenters  It is helpful in locating it  but it does not indicate the datacenter  Therefore it does not work as a URL   So  how do URI  URL  and URN differ in this case    fig  1 is a URI fig  1 is a URN fig  2 is a URI fig  2 is a URL The URL for fig  1 is http   www-example-com s3-website-us-west-2 amazonaws com    also http   www-example-com s3 amazonaws com index html but not http   www-example-com s3 amazonaws com   no datacenter and no filename is too generic for Amazon S3     NOTE  RFC 3986 defines URIs as scheme   authority path query fragment

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This is one of the most confusing and possibly irrelevant topics I ve encountered as a web professional  As I understand it  a URI is a description of something  following an accepted format  that can define both or either the unique name  identification  of something or its location  There are two basic subsets   URLs  which define location  especially to a browser trying to look up a webpage  and URNs  which define the unique name of something   I tend to think of URNs as being similar to GUIDs   They are simply a standardized methodology for providing unique names for things   As in the namespace declarative that uses a company s name   it s not like there is a resource sitting on a server somewhere to correspond to that line of text   it simply uniquely identifies something  I also tend to completely avoid the term URI and discuss things only in terms of URL or URN as appropriate  because it causes so much confusion   The question we should really try answering for people isn t so much the semantics  but how to identify when encountering the terms whether or not there is any practical difference in them that will change the approach to a programming situation   For example  if someone corrects me in conversation and says   quot oh  that s not a URL it s a URI quot  I know they re full of it   If someone says   quot we re using a URN to define the resource  quot  I m more likely to understand we are only naming it uniquely  not locating it on a server  If I m way off base  please let me know

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I found     A uniform resource identifier URI  represents something of a big picture  You can split URIs  URIs can be classified as locators  uniform resource locators- URL   or as names  uniform resource name-URN   or either both  So basically  a URN functions like a person s name and the URL depicts that person s address  So long story short  a URN defines an item s identity  while the URL provides defines the method for finding it  finally encapsulating these two concepts is the URI

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URI is kind of the super class of URL s and URN s  Wikipedia has a fine article about them with links to the right set of RFCs

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Wikipedia will give all the information you need here  Quoting from http   en wikipedia org wiki URI      A URL is a URI that  in addition to identifying a resource  provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network  location

[http] What is the difference between a URI, a URL and a URN?

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