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APPENDIX H:
Network KnowHow
Overview: Network Knowhow
Those of you seeking a more-than-casual knowledge of the way in which networks function will find this
section particularly helpful. The information that follows discusses:
• Ports
• Gateways and Routers
• Subnet Masks
Ports is Ports
So what the heck are ports, anyway? Simply put, they’re part of the addressing that controls how data
travels from one computer to another. You’ve already seen how IP addresses work to identify a single, unique
location somewhere on the Internet, thus enabling you to send packets to a distant computer. But before this can
be done, one more tiny hurdle must be overcome.
A single wire connects the network to the distant computer, but there may be many applications on that
machine—a web server, an ftp server, a telnet server, etc.—waiting for somebody to connect. So the question
arises: How do you use one wire and one IP address to connect to the right application? The answer: Ports.
Here’s an analogy. Let’s say you send a package to a friend. The Zip Code you include on the mailing
label is similar to an IP address in that it gets your package to the one and only town in which your friend lives.
Once there, however, the package still needs to get to your friend’s doorstep. That’s where the street address—
akin to a port number—comes in. In other words, an IP address connects you to the right computer; a port
number lands you in the right application. Computers have many ports (about 65,000), and each has a unique
number.
So, let’s say a network card, listening on its network, has accepted packets intended for its IP address. The
port, a kind of junior-level address, tells the computer who (i.e., which local application) gets which packets.
Kinds of Ports: Source Ports, Destination Ports, Listening Ports, Standard Ports, Ephemeral Ports and
Proxy Ports.
An important principle behind the Internet’s smooth functioning is that some common applications always
have the same ports. In other words, you needn’t know in advance which port on a distant machine accepts
mail, because every machine everywhere uses port 25 to accept email transport. If a mail server application is
running on a machine and is ready to accept mail, the server application opens port 25 and listens for incoming
mail packets.
Some common standard ports:
Telnet
Port 23
Mail (smtp, or send mail)
Port 25
World Wide Web
Port 80
Post Office (pop, or get mail)
Port 110
news (nntp)
Port 119
201