Can't see local domain names from within lan?

My friend is hosting a few personal websites off a computer in his LAN. Now from outside I can access xxx.com and yyy.net (examples obviously) perfectly fine. Accessing localhost from within the network works as well. Everything else works (like google or any other domain name so its not a DNS problem) but for some reason you can't access yyy.net or xxx.com from within the lan (the sites hosted inside the lan) even though the router is forwarding ports correctly (hence me being able to see the sites from outside)

Its a d-link router so I don't know if its a router problem.. never seen anything like this before..

he's also running win2k

any suggestions?

(he just wants to be able to access xxx.com and yyy.net from computers on his lan)

TIA