Move files from external server to server behind router and local ip

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http://www.linuxquestions.org – Hi I have had my web server set up on a machine at home behind a router with a local address 192.168 etc and the router forwards port 80 to the server. I have created a web server on an external host site that has a real ip 198.46.147 etc. I have been able to move files from my 192.168 server to the external one but I am unable to move files to the one behind the router using the same commands I have been using scp to transfer the files but don't know how to find the ip of the pc behind the router ? Any help would be appreciated. External system is Debian 6 Internal system is Centos 6 (HowTos)