reverse resolution of the ip

[steam]

Hello,

I'm running apache on a nix platform.
I would like to assign a server name (i.e. myprimarydomain.com) on a server running many name based domains.

I can't understand the method used to create the reverse resolution on the IP address. For example when I make a traceroute on the ip the resolved host is one of the domains on the server.

Suggestions?

Thanks.