What is IPV6 for localhost and 0 0 0 0

Question

As we all know the IPv4 address for localhost is 127 0 0 1  loopback address   What is the IPv6 address for localhost and for 0 0 0 0 as I need to block some ad hosts

User · Answer

The ipv6 localhost is   1  The unspecified address is     This is defined in RFC 4291 section 2 5

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Just for the sake of completeness  there are IPv4-mapped IPv6 addresses  where you can embed an IPv4 address in an IPv6 address  may not be supported by every IPv6 equipment    Example  I run a server on my machine  which can be accessed via http   127 0 0 1 19983 solr  If I access it via an IPv4-mapped IPv6 address then I access it via http      ffff 127 0 0 1  19983 solr  which will be converted to http      ffff 7f00 1  19983 solr

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For use in a  etc hosts file as a simple ad blocking technique to cause a domain to fail to resolve  the 0 0 0 0 address has been widely used because it causes the request to immediately fail without even trying  because it s not a valid or routable address  This is in comparison to using 127 0 0 1 in that place  where it will at least check to see if your own computer is listening on the requested port 80 before failing with  connection refused   Either of those addresses being used in the hosts file for the domain will stop any requests from being attempted over the actual network  but 0 0 0 0 has gained favor because it s more  optimal  for the above reason   127  IPs will attempt to hit your own computer  and any other IP will cause a request to be sent to the router to try to route it  but for 0 0 0 0 there s nowhere to even send a request to   All that being said  having any IP listed in your hosts file for the domain to be blocked is sufficient  and you wouldn t need or want to also put an ipv6 address in your hosts file unless -- possibly -- you don t have ipv4 enabled at all  I d be really surprised if that was the case  though  And still though  I think having the host appear in  etc hosts with a bad ipv4 address when you don t have ipv4 enabled would still give you the result you are looking for which is for it to fail  instead of looking up the real DNS of say  adserver-example com and getting back either a v4 or v6 IP

User · Answer

As we all know that IPv4 address for localhost is 127 0 0 1  loopback address     Actually  any IPv4 address in 127 0 0 0 8 is a loopback address   In IPv6  the direct analog of the loopback range is   1 128   So   1  long form 0 0 0 0 0 0 0 1  is the one and only IPv6 loopback address     While the hostname localhost will normally resolve to 127 0 0 1 or   1  I have seen cases where someone has bound it to an IP address that is not a loopback address   This is a bit crazy     but sometimes people do it    I say  this is crazy  because you are liable to break applications assumptions by doing this  e g  an application may attempt to do a reverse lookup on the loopback IP and not get the expected result   In the worst case  an application may end up sending sensitive traffic over an insecure network by accident     though you probably need to make other mistakes as well to  achieve  that     Blocking 0 0 0 0 makes no sense   In IPv4 it is never routed   The equivalent in IPv6 is the    address  long form 0 0 0 0 0 0 0 0      which is also never routed   The 0 0 0 0 and    addresses are reserved to mean  any address    So  for example a program that is providing a web service may bind to 0 0 0 0 port 80 to accept HTTP connections via any of the host s IPv4 addresses   These addresses are not valid as a source or destination address for an IP packet     Finally  some comments were asking about    128 versus    0 versus      What is this difference   Strictly speaking  the first two are CIDR notation not IPv6 addresses   They are actually specifying a range of IP addresses   A CIDR consists of a IP address and an additional number that specifies the number of bits in a netmask    The two together specify a range of addresses  i e  the set of addresses formed by ignoring the bits masked out of the given address   So       means just the IPv6 address 0 0 0 0 0 0 0 0    128 means 0 0 0 0 0 0 0 0 with a netmask consisting of 128 bits   This gives a network range with exactly one address in it     0 means 0 0 0 0 0 0 0 0 with a netmask consisting of 0 bits   This gives a network range with 2128 addresses in it   i e  it is the entire IPv6 address space      For more information  read the Wikipedia pages on IPv4  amp  IPv6 addresses  and CIDR notation    https   en wikipedia org wiki IPv4 address https   en wikipedia org wiki IPv6 address https   en wikipedia org wiki Classless Inter-Domain Routing

User · Answer

IPv6 localhost    1 is the loopback address in IPv6    Within URLs  Within a URL  use square brackets        http      1  Defaults to port 80  http      1  80 Specify port    Enclosing the IPv6 literal in square brackets for use in a URL is defined in RFC 2732     Format for Literal IPv6 Addresses in URL s

[localhost] What is IPV6 for localhost and 0.0.0.0?

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