“Most of us are used to referring to ports as the physical
connections on a computer, such as the printer port, USB port,
keyboard port, and so on. But there are logical ports on your
system as well, and technologies such as TCP/IP and UDP networking
use these ports for communication. Port 80, for example, is the
default port for HTTP communications (the Web); whenever you type a
URL such as www.pcmag.com, you’re telling the browser to
communicate through that port. In fact, all Internet protocols
communicate through ports.”
“It doesn’t take long to figure out the security problem
with these port technologies: If a port lets data flow out, it also
lets data flow in. A port is essentially an opening into your
computer, and it can be hacked. Someone can infect your machine
with a Trojan horse in this way, and that’s only one of a host of
distressing possibilities. If you open your computer to the outside
world, you’re vulnerable to attack — period.”
“Port-scanning software tests your network to determine which
TCP ports are vulnerable to attack. These programs analyze typical
TCP ports by default, including HTTP (port 80), Finger (79), FTP
(21), NNTP (119), POP3 (110), SMTP (25), and Whois (43); you can
specify additional ports if you use them (8080 for a Web server,
for example). Depending on the program, you can scan individual IP
addresses or ranges. Keep in mind that scans take a widely varying
amount of time to complete, from seconds to hours.”