I've only done a direct serial connection with a terminal program like Z-Term or the Communications mode of ClarisWorks. In that situation, you'd need to select Null Modem Cable from the list of modems.

If you are using PPP, then you would also have to select Null Modem Cable. The "server" computer would have to have some software running to accept the connection and assign the client an IP address. I'm not sure how that would be done.