Manual:$wgHTCPMulticastAddress

Details
Deprecated. Setting first appeared in 1.4.1, but was commented out in DefaultSettings.php. Set to false from 1.9.0. Replaced by $wgHTCPMulticastRouting in MediaWiki 1.20 and $wgHTCPRouting in MediaWiki 1.22 and subsequent versions.

HTCP is a method of purging entries from the Squid cache that can use multicast and udp which is faster then opening an http connection to every squid server.

The $wgHTCPMulticastAddress variable is deprecated as of MediaWiki 1.20.0 but is still recognised by being copied to the new one in includes/Setup.php: