It is in there already,I think it is different from computer to computer what place in the array your standard ip is (192.168.1.1).
Loop through it and you will find.
IPAddress[] IPAddr = Dns.GetHostEntry(Dns.GetHostName()).AddressList;
for (int i = 0; i < IPAddr.Length; i++)
Console.WriteLine(IPAddr[i].ToString());
On my laptop the ipv4 adress was on
IPAddr[2]