For example, a firewall might be configured to filter out (not forward)
all packets that match the following description :
h 192.12.13.14, 1234, 128.7.6.5, 80 i
This pattern says to discard all packets from port 1234 on host
192.12.13.14 addressed to port 80 on host 128.7.6.5. (Port 80 is the wellknown
TCP port for HTTP.) Of course, it’s often not practical to name
every source host whose packets you want to filter, so the patterns can
include wildcards. For example,