Broadcaster / Seeker basics for sharing and joining games on your local network. Example code for "The implicit local network interface" blog post on AngryAnt.com

Broadcaster.cs

if (m_BroadcastTimer == null)
// Setup broadcast of hosting service
{
	m_BroadcastSocket = new Socket (AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);

	try
	{
		IPAddress broadcastGroup = IPAddress.Parse (kBroadcastGroup);

		m_BroadcastSocket.SetSocketOption (
			SocketOptionLevel.IP,
			SocketOptionName.AddMembership,
			new MulticastOption (broadcastGroup)
		);

		m_BroadcastSocket.SetSocketOption (
			SocketOptionLevel.IP,
			SocketOptionName.MulticastTimeToLive,
			1
		);

		m_BroadcastSocket.Connect (new IPEndPoint (broadcastGroup, kBroadcastPort));
	}
	catch (Exception e)
	{
		Debug.LogError (string.Format (
			"Exception when attempting to set up service broadcast ({0}:{1}): {2}",
			kBroadcastGroup,
			kBroadcastPort,
			e
		));

		return false;
	}

	m_BroadcastTimer = new Timer (kBroadcastDelay);

	m_BroadcastTimer.Elapsed += new ElapsedEventHandler (
		(object s, ElapsedEventArgs e) =>
		{
			m_BroadcastSocket.Send (GetBroadcastBytes ());
		}
	);
	m_BroadcastTimer.Enabled = true;
}

return true;

// ...

byte[] GetBroadcastBytes ()
{
	/*
		Return byte array containing IP and port of our host socket plus meta data
		such as current level and perhaps location (for portal placement)
	*/
}

Seeker.cs

if (m_SeekerSocket == null)
{
	m_SeekerSocket = new Socket (AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);

	try
	{
		m_SeekerSocket.Bind (new IPEndPoint (IPAddress.Any, kBroadcastPort));

		IPAddress broadcastGroup = IPAddress.Parse (kBroadcastGroup);

		m_SeekerSocket.SetSocketOption (
			SocketOptionLevel.IP,
			SocketOptionName.AddMembership,
			new MulticastOption (broadcastGroup, IPAddress.Any)
		);

		ReadBroadcasts ();
	}
	catch (Exception e)
	{
		Debug.LogError (string.Format (
			"Exception when attempting to set up service listener ({0}:{1}): {2}",
			kBroadcastGroup,
			kBroadcastPort,
			e
		));

		m_SeekerSocket.Dispose ();
	}
}

// ...

void ReadBroadcasts ()
{
	/*
		Now set up periodic or event-driven reading from m_SeekerSocket, parsing
		broadcast packages containing IP, port and meta data of locally available games
	*/
}