You can use GetComputerName (or gethostname) and gethostbyname 
(see WINSOCK2.H) for this. Remember that a computer can have 
more than one IP address (it can have multiple adapters, or be 
supporting extra addresses for RAS purposes).

HOSTENT* phe ;
char     szCpuName [128] ;
DWORD    dwSize = sizeof(szCpuName);
int      iaddr, ifield;
UINT     uField;
CString  csIpAddr;
CString  csAddrField;

if (GetComputerName (szCpuName, &dwSize) == 0)
{
   wsprintf (szCpuName, "Can't get computer name, error %d",
   GetLastError ());
   AfxMessageBox (szCpuName);
}
else
{
   phe = gethostbyname (szCpuName);

   if (phe != NULL)
   {
      for (iaddr=0; phe->h_addr_list[iaddr] != NULL; iaddr++)
      {
         csIpAddr = "";
         for (ifield=0; ifield < phe->h_length; ifield++)
         {
            if (ifield > 0)
               csIpAddr += ".";

            uField = ((BYTE*)phe->h_addr_list[iaddr])[ifield];
            csAddrField.Format("%u", uField);
            csIpAddr += csAddrField;
         }
         // Do what you want with the address here. I add it to a listbox.
         m_ctrlListbox.AddString (csIpAddr);
      }
   }
}