Free hostnames are deleted after 30 days of inactivity. Inactivity is defined as when a host has not been updated or 'touched' via the web interface or a dynamic update client.
The client will only submit an update should the IP address change. Basically the client asks our update server what your current IP address is, to which it then checks that IP against previous checks. If the IP is different than the previous check it then submits it back to the update server as a new IP for your selected hosts. Otherwise, if the IP is the same as the last check it then waits until the next check to begin the process again.
When a host is deleted due to inactivity it's not immediately removed from our system. Recently deleted free hostnames are moved into a 30 day redemption period where they cannot be re registered.
After 30 days have passed the host is available for anyone to register. You will need to login to your No-IP account, click the Hosts/Redirects tab and add the hostname again.
We suggest upgrading to the No-IP Enhanced service which will remove this limitation of the free service. The Enhanced benefits are as follows:
1) No more update requirement to keep the hosts active.
2) Access to the No-IP Phone support.
3) Ability to create 25 hosts.
4) More domains to choose from when creating a host.