Firewalls filter based on IP, TCP, and UDP information, among other
things. They are configured with a table of addresses that characterize the
packets they will, and will not, forward. By addresses, we mean more than
just the destination’s IP address, although that is one possibility. Generally,
each entry in the table is a 4-tuple: It gives the IP address and TCP (or
UDP) port number for both the source and destination.