what you need is a router/switch/hub with RS232 port.
although most will tell you that port is for configuring router, there are some devices out there that use it as dial-up and similar.
(back then some LANs were connecting to internet with dialup!)
IIRC, alternatively you could hack palm pilot cradle. (there is a peripheral for old palm that gives it Ethernet over RS232)
both solutions are much harder to find and more expensive than nullmodem cable, though.
