SDL_net, a part of SDL, is a small sample cross-platform networking library, with a sample chat client and server application. It is available under the GNU Library General Public License. SDL_net gives a simple and portable interface for TCP and UDP protocols.
For UDP sockets, SDL_net give an half-way connection: you can bind a socket to an address, avoiding the need of filling outgoing packets with the destination address. This is done using a channel number for the connection. Of course the classic connectionless method is also provided.