This document discusses what is necessary to use PPP over Ethernet (PPPoE) with Mac OS 9.
Mac OS X has PPP over Ethernet (PPPoE) capabilities built-in. If you startup the computer using Mac OS 9 and want to connect to the Internet and your ISP requires PPPoE, then you will need to obtain PPPoE software from your ISP. One example of PPPoE software is discussed in technical document 60645 "Mac OS 9: Is Compatible With MacPoET 1.1."
If you are unable to acquire the PPPoE software and have an AirPort Base Station, then you may still be able to connect under Mac OS 9. AirPort 1.3 and later does support PPPoE. However, if you do not have the Mac OS 9 PPPoE software then the AirPort Base Station must be configured under Mac OS X first.