Blocking websites on lan

I have three computers and i want to block porn sites from main computer tell me how can i block them from the the server
THANX

If its windows use the parental control thats built-in the os.

The link didn't work, so go to http://www.download.com and use "parental control" as search string.
Install one of those progs...

..... and if it's a *nix based server you could look at Dansguardian (www.dansguardian.org) or SquidGuard (www.squidguard.org) - either of these can block porn sites as well as many other things.

I use Dansguardian here (running on an IPCop firewall computer) to restrict the kids from porn sites - as you will never completely block every porn site - and from downloading files that are either going to blow my bandwidth quota, get me into trouble with MPAA, RIAA type people or introduce nasties into the network. (.exe, .scr, .pif etc..)