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.because I need to get these had two values programmatically in order to send a broadcast message to my different 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.

Answer 1

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 & myIP)
{
    char szBuffer[1024];

    #ifdef WIN32
    WSADATA wsaData;
    WORD wVersionRequested = MAKEWORD(2, 0);
    if(::WSAStartup(wVersionRequested, &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->h_addr))->S_un.S_un_b.s_b1;
    myIP.b2 = ((struct in_addr *)(host->h_addr))->S_un.S_un_b.s_b2;
    myIP.b3 = ((struct in_addr *)(host->h_addr))->S_un.S_un_b.s_b3;
    myIP.b4 = ((struct in_addr *)(host->h_addr))->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

Answer 2

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

Answer 3

If you use winsock, here's a way: http://tangentsoft.net/wskfaq/examples/ipaddr.html

Answer 4

Winsock specific:

// Init WinSock
WSADATA wsa_Data;
int wsa_ReturnCode = WSAStartup(0x101,&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->h_addr_list);
WSACleanup();

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

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->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++..

Answer 9

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.