According to EveryMac.Com,
The 7100 did not have a standard RJ-45 Ethernet connector, but rather Apple's AAUI funky connector, which does indeed require an AAUI-RJ45 adapter, which costs about $35, if you can find one. The other plug that almost looks like an ethernet plug is your modem!
You might also want to boost the RAM and VRAM to their maximums (136mb and 2mb, respectively) and put at least OS 7.5.3 on it. It's only a 66mhz machine (or, if it's post-1995, 80mhz), but that's enough for email and... well... email
Also visit Low End Mac for details on your model and other resources / advice: