getting IP of client behind a firewall

[sneakyimp]

i'm working on some code which can obtain the specific IP address of a user of my site. the problem i'm having is distinguishing between computers behind a firewall. When i access this script on my server:

<?
echo 'REMOTE_ADDR:' . $_SERVER['REMOTE_ADDR'] . '<br>';
echo 'HTTP_X_FORWARDED_FOR:' . $_SERVER['HTTP_X_FORWARDED_FOR'] .
'<br>';
echo 'HTTP_CACHE_CONTROL:' . $_SERVER['HTTP_CACHE_CONTROL'] .
'<br>';
?>

only the first line returns an address. my computer is among 3 computers behind my firewall. Is there any way to distinguish them?

[Wynder]

Negatory...

Think of it kinda like a home network -- you have three machines... 192.168.1.1, 192.168.1.2, 192.168.1.3... those are connected to a firewall/router which is assigned an IP address by your ISP -- you'll only see those machines are 'masquarading' as that one IP address because you can only see the foremost link in the chain.

Kinda the same deal with proxies.

[sneakyimp]

the php docs (and most of the comments) suggest that you can get that last-mile IP info for people behind a proxy from $_SERVER['HTTP_X_FORWARDED_FOR'].