To answer my own question, it's in the stdc.bmx module, which is included by default. See the socket_ function.
[Edit]
"socket_" is the low level function. For the easy way to write a network client, see the SocketStream module. For an example, see the HttpStream module. I wrote up a working example last night, communicating with a server written in Rebol. It took a couple of lines of code each for the client and server.
[Edit]
"socket_" is the low level function. For the easy way to write a network client, see the SocketStream module. For an example, see the HttpStream module. I wrote up a working example last night, communicating with a server written in Rebol. It took a couple of lines of code each for the client and server.