Make sure you have the latest release of Open Transport. See the Apple Software Updates Archive for updates. Leave the IP address , Subnet mask , and Router address fields blank. Configure MacSLIP normally. MacSLIP will obtain the addresses using Bootp, the PPP address negotiation, or from your script using the ipfind command.
Case is significant.
When using Open Transport, you should only have one serial transport installed. If you have installed MacSLIP, you should remove other versions of FreePPP, InterSLIP, or other serial transports. You may have to connect with the MacSLIP control panel or MacSLIP menu. If you connect to a server that assigns you a new IP address every time you connect, you may have to reboot before reconnecting. This is an Open Transport limitation that should be fixed in version 1.1 of Open Transport.