Restricting access to internal hosts

Restricting access to internal hosts

hi all!

i have a problem which i hope you can help me with. please see image located
he

http://img208.imageshack.us/img208/5421/networkq.jpg

as you can see i connected a router with the company network and which
routes to a gateway connected to the internet. everything is working ok and
there is full connectivity throughout. my problem is that users which are
connecting to the internet via the router are able to access the internal
workstations. is there a way how i could still allow internet access to the
machines connected to the router but disallow them access to the internal
workstations?

thanks!