This section describes examples for configuring the security appliance to support Dynamic DNS. DDNS update integrates DNS with DHCP. The two protocols are complementaryDHCP centralizes and automates IP address allocation, while dynamic DNS update automatically records the association between assigned addresses and hostnames. When you use DHCP and dynamic DNS update, this configures a host automatically for network access whenever it attaches to the IP network. You can locate and reach the host using its permanent, unique DNS hostname. Mobile hosts, for example, can move freely without user or administrator intervention.

Refer to the document which gives more information on DDNS

http://www.cisco.com/en/US/docs/security/asa/asa80/configuration/guide/dhcp.html#wp1091527