Tcp/ip filter problem

Hi
I have a windows 2000 server Sp4 with 2 NIC one connected on lan and the
other one connected on wan. I create a Tcp/ip filter that block all traffic
over internet from my lan and enable only a specif ip to pass. Tha other
client to try to connect using as default gateway this server ip are
redirect to another ip.
This filter was configured to use all my subnet as ip, and work fine, now i
see that everyone can use this server to go on internet.
I try to modify the filter and if i insert the single ip to block in the
properties it's work but if i insert a specific subnet don't work again.
Any idea about.
TIA Pupo