Nathan, i don't trust that a set top box won't be NAT capable or enabled, please open a command promt (start > all programs > Accessories > command prompt)& type /ipconfig
if yor ip address shown there falls within any of the ranges below, your set top box IS acting as a router for these purposes, don't be fooled by it only having one socket to plug into, many of the modem/router combos we frequently deal with in the help room only have one socket..
Private IP Ranges:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
the first & last of these are quite commonly used by NAT modems & routers, the middle one less commonly, but it's not unheard of.
In either case posting the make & model of the set top box may be helpful.