How to get started: sudo tcpdump
The next statement you must heed to, it is not a joke: Do *not* use packet
sniffing while logged into a net bank, you might get punished for it!
Also, port scanning is only allowed for your own hosts, otherwise it is
illegal. If you try to port scan the government's or the bank's servers, the
police might have to come to get you! tcpdump is not a port scanner, nmap is.
one can learn to use
the nmap program with the command: info nmap