I didn't understand your post ... but I'm going to explain a lil bit closer ...
if you're going into the internet through a proxy (like router, server or home sharing) only the Proxy has an internet IP ... all others are just in a network with it ... so your PC has NO internetIP
then there're ports in IP system ... think in a wall with 65536 holes
... every connection thing is using one ... 0-1023 are reserved (20-21 for FTP .....) so 1024-65535 are free choosable ... HL uses (ordinary (it's changeable)) port 27005 for client and 27015 for servers
so you've to tell the proxy that this port should be linked to your PC ... then if you're connecting to this IP with this port you're not connected to te proxy anymore ... you're connected to the pc
if you don't know howto link ports, just IM me ... I think I can help you (cause I'm behind a router, too)