i have a private lan with server (Win2003) and clients (WinXP).

i put 3 local sites on the server (under inetpub directory). all clients can accsess to any site by using the server name/IP followed by site folder name. (http://server_name/sitefoldername)

my qustion:
How to assign a unique domain name for each site ?

for example: http://www.any_domain.com

note: our LAN is not connected with internet.

Thanks for help.

Are you running Active Directory? If so then you're running DNS services. You can create custom DNS domains within the AD integrated DNS and then point them to the necessary IP addresses.