capture IP that are behind a firewall

I mininterpretted the question. As Dan says, if the server and the client are behind the SAME firewall, this will work. If your network is using static IPs, this will prove to be quite reliable.

For some reason, I jumped to the conclusion that you were attempting to retrieve the IP from behind someone else's firewall.

If your server is behind the same firewall, the ip address is available.

This is for IP range from a specific network witihin a firewall.

Just out of curiosity, what would you do withthe IP address once you had it? With the common use of dynamic IP addresses nowadays, it seems hard to picture a reliable usage for recording the IP.

nope...cgi.remote_addr would return the external interface ip of the firewall. Because CF executes on the web server and not the client then you won't be using CF - maybe javascript but i'm not sure if that has access to the IP level of info,