IPv6 Specific Content in Apache

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In PHP you can take different actions based on whether the client is connecting from IPv4 or IPv6. This is an example from Linux Blog: 

<?php
$from=$_SERVER["REMOTE_ADDR"];
$ipv6="off";
$message="Our website is available on IPv6. Get connected!";
if (ereg("^[23]...[:]", $from))
       {
       $ipv6="on";
       $message="Our connectivity is powered by SixXS!";
       }
?>
<A HREF="http://www.sixxs.net" target="_blank">
<IMG SRC="http://blogimages.project76.tv/ipv6_<?php echo $ipv6; ?>.png"
WIDTH="96" HEIGHT="32" ALT="<?php echo $message; ?>"></A>

This file is then included in the blog template with 

<!--#include virtual="/ipv6_detect.php" -->

The remote client address is checked; because all valid IPv6 space is currently in 2000::/3 the first 16-bit segment will always have 4 digits in its representation, will start with a 2 or a 3, and will have a colon in position 5. The regular expression exploits this fact.

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