PC Magazine: Freeware Port Scanners: Plug the HolesNov 17, 2000, 22:14 (0 Talkback[s])
(Other stories by Neil Randall)
"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."