Determine inbound -vs- outbound by the port number

[Baron Schwartz]

Instead of looking for traffic to an interface by inspecting IP addresses, when -p is given we should look only at the port number. If the packet's destination port matches the -p port, the packet is inbound, otherwise outbound. This will make it possible to trace traffic easily from a different server. For example, if I log into a web server and want to measure traffic from the web server to the database server, I should be able to do that.