Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 525)

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Firewall Service
Firewall service is software that protects the network applications running on your
Mac OS X Server. Turning on Firewall service is similar to erecting a wall to limit access.
Firewall service scans incoming IP packets and rejects or accepts these packets based on the
set of filters you create. You can restrict access to any IP service running on the server, and
you can customize filters for all incoming clients or for a range of client IP addresses.
Services such as Web and FTP are identified on your server by a Transmission Control
Protocol (TCP) or User Datagram Protocol (UDP) port number. When a computer tries to
connect to a service, Firewall service scans the filter list for a matching port number.
m If the port number is in the filter list, the filter applied is the one that contains the most
specific address range.
m If the port number is not in the list, the Any Port filter that contains the most specific
address range is used.