yep. System 6, System 7.x, and MacTCP work fine for me behind a router with a fixed LAN IP address for the 68K Mac. I've done this with an SE/30, Mac IIci, and Quadra 605, 650, 700, and 800. These machines have been used for LAN file storage, LAN DNS, LAN e-Mail, LAN Intranet, etc. You'd still do most real world surfing on a more modern machine.

The issues are discussed here. This may fix some issues. (but only works with System 7.x, not System 6).