|Take a look for the gateway IP as wanderer said.|
I recommended a SOHO router for a couple of reasons:
1) They're pretty cheap these days
2) They come with a builtin Firewall as well as NAT and DHCP
3) Your present setup means you have to have the PC on and running in order for other LAN clients to access the internet. A SOHO router means you don't have to.
If you can afford the $40 to $50 for a SOHO router, I'd definately go that route.