This Gist gets all local available IP addresses

get-all-ips.cs

using System;
using System.Net;
using System.Linq;

namespace tester {
    public static class iptester {
        public static IPAddress GetServerIP() {
            var localIP = getMyAddress();
            var subnetmask = getSubnetMask(localIP);
            for (byte p1 = 1; p1 < subnetmask[0]; p1++) {
                for (byte p2 = 1; p2 < subnetmask[1]; p2++) {
                    for (byte p3 = 1; p3 < subnetmask[2]; p3++) {
                        for (byte p4 = 1; p4 < subnetmask[3]; p4++) {
                            var ip = new IPAdress(p1, p2, p3, p4);
                            if (trytoreach(ip)) {
                                return ip;
                            }
                        }
                    }
                }
            }
        }
        private static IPAddress getMyAddress() {
            // http://stackoverflow.com/a/7661338
            return Dns.GetHostEntry(Dns.GetHostName()).AddressList.FirstOrDefault(ip => ip.AddressFamily == AddressFamily.InterNetwork);
        }
        private static IPAddress getSubnetMask(IPAddress address) {
            // I have no idea how to get the subnet mask
            return default(IPAddress);
        }
        private static bool trytoreach(IPAddress ip) {
            var bytes = ip.GetAddressBytes();
            var req = (HttpWebRequest)WebRequest.CreateHttp(string.Format("http://{0}.{1}.{2}.{3}/iamhere", bytes[0], bytes[1], bytes[2], bytes[3]));
            using (var resp = (HttpWebResponse)req.GetResponse()) {
                if (resp.StatusCode == 200) {
                    return true;
                }
            }
        }
    }
}