First, the normal installed version of Simutrans works as either a client and a server.
For example, on a Linux system:
To start Simutrans normally (client mode):
To start Simutrans as a server:
$ simutrans -server
Second, you can run a client and a server on the same machine. To connect the client to the server, use the loopback address (127.0.0.1) or the machine's IP address in the client's connect dialog.
Now we get into pesky little details:
- The client and server must both be using the same pakset version and addons (if any). The server and client compare hashes of each pakset and addon item, and refuse to connect if every hash doesn't match.
- The client and server should (not must) be using the same game version, or within a release or two. Network play is still pretty new, improvements are still being added, and backward compatibility with older versions is not guaranteed.