Specifying a Default DMZ Server allows you to set up a computer or server that is available to anyone on the Internet for services that you haven't defined. There are security issues with doing this, so only do this if you'll willing to risk open access. If you do not assign a Default DMZ Server, the router discards any undefined service request.
To assign a computer or server to be a DMZ server:
To remove a computer or server from being a DMZ server: