FireWall for Dial-In Users

[otaibi @ sapic . com]

Hello everyone,
    Is it possible to have a firewall to protect the private network
from the dial-in users ?
We have an internal LAN allowing some users to dial-in to our interanet
(using Windows NT RAS) and thinking to have a firewall to protect our
LAN.
    Does firewalls work with modems as external interfaces instead of
the normal ethernet interfacs ?
If yes, how it will be done ? I appreciate any help.
Thanks all & regards,
Mr. Hamoud Otaibi.