When setting up firewalls on your local network, be sure to configure them in such a way that allows computers running Mac OS X and Mac OS X Server to communicate over UDP port 626. Local network firewalls may be configured on the computers themselves or on the Ethernet network.
Some Apple software products use this port to check whether any other computers on the same (local) network are using the same serial number. If port 626 is blocked, then certain services may not start as expected, or you may not be able to use some applications.
It is not necessary to leave UDP port 626 open on firewalls that connect a private network to the Internet.