Get the IP Address of local computer

Question

In C    what s the easiest way to get the local computer s IP address and subnet mask   I want to be able to detect the local machine s IP address in my local network  In my particular case  I have a network with a subnet mask of 255 255 255 0 and my computer s IP address is 192 168 0 5  I need to get these had two values programmatically in order to send a broadcast message to my network  in the form 192 168 0 255  for my particular case   Edit  Many answers were not giving the results I expected because I had two different network IP s  Torial s code did the trick  it gave me both IP addresses   Thanks     Edit 2  Thanks to Brian R  Bondy for the info about the subnet mask

User · Answer

The question is trickier than it appears, because in many cases there isn't "an IP address for the local computer" so much as a number of different IP addresses. For example, the Mac I'm typing on right now (which is a pretty basic, standard Mac setup) has the following IP addresses associated with it:

fe80::1%lo0  
127.0.0.1 
::1 
fe80::21f:5bff:fe3f:1b36%en1 
10.0.0.138 
172.16.175.1
192.168.27.1

... and it's not just a matter of figuring out which of the above is "the real IP address", either... they are all "real" and useful; some more useful than others depending on what you are going to use the addresses for.

In my experience often the best way to get "an IP address" for your local computer is not to query the local computer at all, but rather to ask the computer your program is talking to what it sees your computer's IP address as. e.g. if you are writing a client program, send a message to the server asking the server to send back as data the IP address that your request came from. That way you will know what the relevant IP address is, given the context of the computer you are communicating with.

That said, that trick may not be appropriate for some purposes (e.g. when you're not communicating with a particular computer) so sometimes you just need to gather the list of all the IP addresses associated with your machine. The best way to do that under Unix/Mac (AFAIK) is by calling getifaddrs() and iterating over the results. Under Windows, try GetAdaptersAddresses() to get similar functionality. For example usages of both, see the GetNetworkInterfaceInfos() function in this file.

User · Answer

You can use gethostname followed by gethostbyname to get your local interface internal IP     This returned IP may be different from your external IP though   To get your external IP you would have to communicate with an external server that will tell you what your external IP is   Because the external IP is not yours but it is your routers     Example  b1    192  b2    168  b3    0  b4    100 struct IPv4       unsigned char b1  b2  b3  b4      bool getMyIP IPv4  amp  myIP        char szBuffer 1024         ifdef WIN32     WSADATA wsaData      WORD wVersionRequested   MAKEWORD 2  0       if   WSAStartup wVersionRequested   amp wsaData     0          return false       endif       if gethostname szBuffer  sizeof szBuffer      SOCKET ERROR               ifdef WIN32       WSACleanup           endif       return false             struct hostent  host   gethostbyname szBuffer       if host    NULL               ifdef WIN32       WSACleanup           endif       return false               Obtain the computer s IP     myIP b1     struct in addr    host- gt h addr  - gt S un S un b s b1      myIP b2     struct in addr    host- gt h addr  - gt S un S un b s b2      myIP b3     struct in addr    host- gt h addr  - gt S un S un b s b3      myIP b4     struct in addr    host- gt h addr  - gt S un S un b s b4        ifdef WIN32     WSACleanup         endif     return true      You can also always just use 127 0 0 1 which represents the local machine always   Subnet mask in Windows   You can get the subnet mask  and gateway and other info  by querying subkeys of this registry entry   HKEY LOCAL MACHINE SYSTEM CurrentControlSet Services Tcpip Parameters Interfaces  Look for the registry value SubnetMask   Other methods to get interface information in Windows   You could also retrieve the information you re looking for by using   WSAIoctl with this option  SIO GET INTERFACE LIST

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In DEV C    I used pure C with WIN32  with this given piece of code   case IDC IP                gethostname szHostName  255                host entry gethostbyname szHostName                szLocalIP   inet ntoa    struct in addr    host entry- gt h addr list                  WSACleanup                  writeInTextBox   n                 writeInTextBox  IP                    writeInTextBox szLocalIP                break    When I click the button  show ip   it works  But on the second time  the program quits  without warning or error   When I do     WSACleanup       The program does not quit  even clicking the same button multiple times with fastest speed  So WSACleanup   may not work well with Dev-C

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I was able to do it using DNS service under VS2013 with the following code    include  lt Windns h gt   WSADATA wsa Data   int wsa ReturnCode   WSAStartup 0x101   amp wsa Data    gethostname hostName  256   PDNS RECORD pDnsRecord   DNS STATUS statsus   DnsQuery hostName  DNS TYPE A  DNS QUERY STANDARD  NULL   amp pDnsRecord  NULL   IN ADDR ipaddr  ipaddr S un S addr    pDnsRecord- gt Data A IpAddress   printf  The IP address of the host  s is  s  n   hostName  inet ntoa ipaddr     DnsRecordListFree  amp pDnsRecord  DnsFreeRecordList     I had to add Dnsapi lib as addictional dependency in linker option   Reference here

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Can t you just send to INADDR BROADCAST  Admittedly  that ll send on all interfaces - but that s rarely a problem   Otherwise  ioctl and SIOCGIFBRDADDR should get you the address on  nix  and WSAioctl and SIO GET BROADCAST ADDRESS on win32

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I suggest my code   DllExport void get local ips boost  container  vector lt wstring gt  amp  ips       IP ADAPTER ADDRESSES        adapters    NULL     IP ADAPTER ADDRESSES        adapter         NULL     IP ADAPTER UNICAST ADDRESS  adr             NULL     ULONG                       adapter size   0     ULONG                       err             0     SOCKADDR IN                 sockaddr    NULL      err     GetAdaptersAddresses AF UNSPEC  GAA FLAG SKIP ANYCAST   GAA FLAG SKIP MULTICAST   GAA FLAG SKIP DNS SERVER   GAA FLAG SKIP FRIENDLY NAME  NULL  NULL   amp adapter size      adapters    IP ADAPTER ADDRESSES  malloc adapter size      err     GetAdaptersAddresses AF UNSPEC  GAA FLAG SKIP ANYCAST   GAA FLAG SKIP MULTICAST   GAA FLAG SKIP DNS SERVER   GAA FLAG SKIP FRIENDLY NAME  NULL  adapters   amp adapter size       for  adapter   adapters  NULL    adapter  adapter   adapter- gt Next              if  adapter- gt IfType        IF TYPE SOFTWARE LOOPBACK  continue     Skip Loopback        if  adapter- gt OperStatus    IfOperStatusUp  continue                Live connection only           for  adr   adapter- gt FirstUnicastAddress adr    NULL  adr   adr- gt Next                      sockaddr    SOCKADDR IN   adr- gt Address lpSockaddr              char    ipstr  INET6 ADDRSTRLEN      0               wchar t ipwstr INET6 ADDRSTRLEN      0               inet ntop AF INET   amp  sockaddr- gt sin addr   ipstr  INET ADDRSTRLEN              mbstowcs ipwstr  ipstr  INET6 ADDRSTRLEN              wstring wstr ipwstr              if  wstr     0 0 0 0   ips push back wstr                                           free adapters      adapters   NULL

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Also  note that  the local IP  might not be a particularly unique thing  If you are on several physical networks  wired wireless bluetooth  for example  or a server with lots of Ethernet cards  etc    or have TAP TUN interfaces setup  your machine can easily have a whole host of interfaces

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from torial      If you use winsock  here s a way  http   tangentsoft net wskfaq examples ipaddr html      As for the subnet portion of the question  there is not platform agnostic way to retrieve the subnet mask as the POSIX socket API  which all modern operating systems implement  does not specify this  So you will have to use whatever method is available on the platform you are using

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Also  note that  the local IP  might not be a particularly unique thing  If you are on several physical networks  wired wireless bluetooth  for example  or a server with lots of Ethernet cards  etc    or have TAP TUN interfaces setup  your machine can easily have a whole host of interfaces

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How to Obtain the IP Address of the Local Machine on the Network seems to describe the solution quite well

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The problem with all the approaches based on gethostbyname is that you will not get all IP addresses assigned to a particular machine  Servers usually have more than one adapter   Here is an example of how you can iterate through all Ipv4 and Ipv6 addresses on the host machine   void ListIpAddresses IpAddresses amp  ipAddrs      IP ADAPTER ADDRESSES  adapter addresses NULL     IP ADAPTER ADDRESSES  adapter NULL         Start with a 16 KB buffer and resize if needed -      multiple attempts in case interfaces change while      we are in the middle of querying them    DWORD adapter addresses buffer size   16   KB    for  int attempts   0  attempts    3    attempts          adapter addresses    IP ADAPTER ADDRESSES  malloc adapter addresses buffer size       assert adapter addresses        DWORD error     GetAdaptersAddresses        AF UNSPEC         GAA FLAG SKIP ANYCAST            GAA FLAG SKIP MULTICAST            GAA FLAG SKIP DNS SERVER           GAA FLAG SKIP FRIENDLY NAME         NULL         adapter addresses         amp adapter addresses buffer size        if  ERROR SUCCESS    error                 We re done here  people        break            else if  ERROR BUFFER OVERFLOW    error                 Try again with the new size       free adapter addresses         adapter addresses   NULL         continue            else                Unexpected error code - log and throw       free adapter addresses         adapter addresses   NULL             todo       LOG AND THROW HERE                    Iterate through all of the adapters   for  adapter   adapter addresses  NULL    adapter  adapter   adapter- gt Next             Skip loopback adapters     if  IF TYPE SOFTWARE LOOPBACK    adapter- gt IfType              continue                Parse all IPv4 and IPv6 addresses     for         IP ADAPTER UNICAST ADDRESS  address   adapter- gt FirstUnicastAddress         NULL    address        address   address- gt Next              auto family   address- gt Address lpSockaddr- gt sa family        if  AF INET    family                     IPv4         SOCKADDR IN  ipv4   reinterpret cast lt SOCKADDR IN  gt  address- gt Address lpSockaddr            char str buffer INET ADDRSTRLEN     0           inet ntop AF INET   amp  ipv4- gt sin addr   str buffer  INET ADDRSTRLEN           ipAddrs mIpv4 push back str buffer                 else if  AF INET6    family                     IPv6         SOCKADDR IN6  ipv6   reinterpret cast lt SOCKADDR IN6  gt  address- gt Address lpSockaddr            char str buffer INET6 ADDRSTRLEN     0           inet ntop AF INET6   amp  ipv6- gt sin6 addr   str buffer  INET6 ADDRSTRLEN            std  string ipv6 str str buffer               Detect and skip non-external addresses         bool is link local false           bool is special use false            if  0    ipv6 str find  fe                        char c   ipv6 str 2             if  c     8     c     9     c     a     c     b                           is link local   true                                else if  0    ipv6 str find  2001 0                         is special use   true                     if     is link local    is special use                       ipAddrs mIpv6 push back ipv6 str                           else                    Skip all other types of addresses         continue                          Cleanup   free adapter addresses     adapter addresses   NULL        Cheers

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You cannot do that in Standard C     I m posting this because it is the only correct answer  Your question asks how to do it in C    Well  you can t do it in C    You can do it in Windows  POSIX  Linux  Android  but all those are OS-specific solutions and not part of the language standard   Standard C   does not have a networking layer at all   I assume you have this wrong assumption that C   Standard defines the same scope of features as other language standards  Java  While Java might have built-in networking  and even a GUI framework  in the language s own standard library  C   does not   While there are third-party APIs and libraries which can be used by a C   program  this is in no way the same as saying that you can do it in C     Here is an example to clarify what I mean  You can open a file in C   because it has an fstream class as part of its standard library  This is not the same thing as using CreateFile    which is a Windows-specific function and available only for WINAPI

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Winsock specific      Init WinSock WSADATA wsa Data  int wsa ReturnCode   WSAStartup 0x101  amp wsa Data       Get the local hostname char szHostName 255   gethostname szHostName  255   struct hostent  host entry  host entry gethostbyname szHostName   char   szLocalIP  szLocalIP   inet ntoa    struct in addr    host entry- gt h addr list   WSACleanup

[c++] Get the IP Address of local computer

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