I was not misremembering, but I'm not sure what of this exists in 7.5 (I'd guess all of it, but I honestly forget).  I booted to 8.5, restarted into 7.6.  I plugged the AAUI to Ethernet adapter in and it lit up (thankfully suggesting it's plug-and-play).  I plugged the ethernet cable in and opened the AppleTalk Control Panel.

1) Select "Ethernet" in the AppleTalk Control Panel.  Save when closing.

2) Open TCP/IP Control Panel, activate it (click yes when opening).  Make sure "Ethernet" and "DHCP Server" are selected (the latter, unless you want to manually configure as uniserver mentioned).

3) You should be on the internet... I'm trying to get a browser to verify that - I don't know how to ping from Classic Macs.

** I had to reboot to OS 8.5 to try to grab a 7-compatible browser (pending)