I have CS server behind router and I opened the ports, there is no problem for the players to join the server, but all of them obtain the router IP address and if I ban one I ban all... Please help.

Well the problem lies with your router and how it assigns ip address to those who are connected. What you could probably do as a quick solution is to assign ip addresses manually.

I think that you could check the router settings concerning the subnet mask as this would probably be causing it.

But I am pretty sure that if you just setup your router properly, you would have no problems with this.