Dynamic DNS

There are essentially two types of "Dynamic" DNS. The most common non-enterprise use is of services like DynDNS which allows, via the web (port 80/tcp HTTP or port 443/tcp-HTTPS) a method where clients can use PUT or GET calls to trigger the update of DNS records (type A, PTR usually).

The second way is per RFC 2316.

http://doc.pfsense.org/index.php/Dynamic_DNS provides a great explanation of both.