On IRC a vhost (virtual host) used together with a bouncer as a method of masking/hiding the users true hostname/IP address. This is done to ensure privacy, and prevent (DoS) attacks. It's also done to have cool host names, either for yourself or your bot.

Typically vhosts and bouncers are setup and run on shell providers. Typically a shell provider will have a range of vhost names you can use. To find these you usually issue a command like 'vhost' or 'vhosts' in your shell, and the avaiable vhosts are listed. Many shell providers provide custom vhosts, where they supply and IP address, and you supply the domain. You set up an A record (or AAAA record for IPv6), and they set up a PTR record.

If your ISP can delegate reverse DNS for your IP-addresses to you, you can do it all on your own network. You'll need a couple of name server (or at least one primary server; maybe someone else can provide you with a slave). In addition you'll need at least one domain name that you can/want to use for vhosts.

A bouncer is an IRC proxy, the bouncer connects to the IRCD, and you connect your IRC client to the bouncer. The bouncer can be left idelling on IRC even when you are not there.

Vhosts can also provide cool hostnames to bots (like Eggdrop, etc.) and terminal IRC clients that you run directly on the shell (like BitcX, etc.). You can even leave this these terminal clients ideling (when you are not around and logged out of the shell) if you run them thru the program screen.