as has been posted at nearly all Mac-centric websites, there is an Apple-acknowledged DHCP problem with Mac OS 9, or more specifically with Open Transport 2.5.2, which results in a failure to renew IP address leases when using DHCP and erratic behavior when rebinding stale leases. This is why many users of cable modem and DSL are encountering problems with Mac OS 9. http://til.info.apple.com/techinfo.nsf/artnum/n25049/
suggested workaround is downgrading to the Open Transport files from Mac OS 8.6
another report says fixed DHCP problems with Mac OS 9 by trashing TCP/IP Preferences file. Also make sure that TCP/IP's (control panel) 'Load only when needed' is not checked.
these may or may not work for YOU...