I'm not sure how to do this with SSI (or if you can) as it's so old
This can be done very easily on PHP however, just a few line of code to tes the variables and to display them as explained in Paul's link.
E.G.
PHP Code:
<?php
if (getenv(HTTP_X_FORWARDED_FOR)) {
//get past a proxy addy
$ip=getenv(HTTP_X_FORWARDED_FOR);
} else {
//normal ip addy
$ip=getenv(REMOTE_ADDR);
}
echo "$ip";
?>
Some caches did return the IP address backwards at one point, but I believe his was fixed.