I've had some serious problems getting MacTCP to work with my Linksys WRT54GL router. Turns out those Linksys routers (and probably certain other brands as well) perform some kind of reordering of packet priority to improve the quality of service, and MacTCP doesn't like that. I could get MacTCP Ping to work, but anything more complicated than that (Fetch, MacWeb, Mosaic, whatever) just wouldn't work.
There are two solutions:
1) Use OpenTransport instead of MacTCP. That'll work perfectly with a 12 MB Color Classic and System 7.1, even if it is a bit of a memory hog. You'll also have the added advantage of DHCP support, so you don't have to manually set your IP address.
2) Patch MacTCP to the unofficial version 2.1, as found here. That fixed the Linksys incompatibility for me, and works great on my 4 MB Classic.
Of course I cannot guarantee that you're having the same problem, but it's worth a try at least.
There are two solutions:
1) Use OpenTransport instead of MacTCP. That'll work perfectly with a 12 MB Color Classic and System 7.1, even if it is a bit of a memory hog. You'll also have the added advantage of DHCP support, so you don't have to manually set your IP address.
2) Patch MacTCP to the unofficial version 2.1, as found here. That fixed the Linksys incompatibility for me, and works great on my 4 MB Classic.
Of course I cannot guarantee that you're having the same problem, but it's worth a try at least.