Appendix D: Understanding port forwarding
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So where do ports come into this process? Ports are an old but useful holdover from the early days
of network computing. Back in the day, when computers could only run one application at a time,
all you had to do was point one computer at another computer on the network to connect them as
they would be running the same application. Once computers became sophisticated to run multiple
applications, early computer scientists had to wrestle with the issue of ensuring applications
connected to the right applications. Thus, ports were born.
Some ports have specific applications which are standards throughout the computing industry.
When you fetch a web page, for example, it uses port 80. The receiving computer’s software knows
that port 80 is used for serving http documents, so it listens there and responds accordingly. If you
send an http request over a different port—say, 143—the web server won’t recognize it because it’s
not listening there (although something else might be, like an IMAP email server which traditionally
uses that port).
Other ports don’t have pre-assigned uses, and you can use them for whatever you want. To avoid
interfering with other standard-abiding applications, it’s best to use larger numbers for these
alternate configurations. Plex Media Server uses port 32400, for example, and Minecraft servers use
25565—both numbers that fall into this “fair game” territory.
Each port can be used via either TCP or UDP. TCP, or Transmission Control Protocol, is what’s
used most commonly. UDP, or User Datagram Protocol, is less widely used in home applications
with one major exception: BitTorrent. Depending on what is listening, it’ll be expecting requests to
be made in either one or the other of these protocols.
Why you need to forward ports
So why exactly would you need to forward ports? While some applications take advantage of
NAT to set their own ports and handle the configuration for you, there are still plenty of
applications that do not, and you’ll need to give your router a helping hand when it comes to
connecting services and applications.
In the diagram below we’re starting with a simple premise. You’re on your laptop somewhere in the
world (with an IP address of 225.213.7.42), and you want to connect to your home network to
access some files. If you simply plug your home IP address (127.34.73.214) into whatever tool you’re
using (an FTP client or remote desktop application, for example), and that tool doesn’t take
advantage of those advanced router features we just mentioned, you’re out of luck. It won’t know
where to send your request, and nothing will happen.
Summary of Contents for AutoFlex Connect
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