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One of the major
components (if you can call it
that) of the internet is
TCP/IP.
If the internet were a body, then
TCP/IP
would be the nerve impulses which make it do things (that's a very rough
analogy, but it will serve).
A TCP/IP number
is a set of four smaller numbers, separated by dots, which represent the
address of a single machine on the internet.
Every single machine on a
TCP/IP
network has a unique number which other machines can use to send messages to
that machine.
Thus to send, say, a web
page to a machine a web server might send it to a
TCP/IP
address of
167.34.23.1.
How does it know where to send it?
The machine which
asked for the web page (the user entered a URL in a browser) sent along it's
address in the request. The web server merely sends the data back to that
address.
TCP/IP
Transmission Control Protocol / Internet Protocol. The primary
protocol used on the Internet.
Here's where port
numbers come into play.
Let's say you
want to look at a web page and you enter the URL into your browser. This gets
sent out to the internet, and through various means winds up at a machine
which receives browser requests and returns graphics, web pages and perhaps
video and sound.
That machine
probably does a number of things. Of course, it handles requests
for web pages and such, but it also must send email (using a program called
sendmail), receive mail (using POP or Post Office Protocol)....
.... responds to PING
requests (a way to determine if a machine is up or has crashed), talk to the
name servers so that the internet knows it exist and any of a hundred other
things. (Protocol - A method of sending and receiving data)
POP
Post Office Protocol
rfc1939 used for delivering messages to (client) mail readers
(fetches mails from a
remote mail server and stores it locally).
How does
the machine know the difference?
The answer is a port
number is used. Each of the services
that the machine performs is done by a program. Each program does something
called listening on a port.
A port number is actually a
number between 1 and 65535
which identifies to the receiving computer what function you want to
perform.
Your web browser
normally makes requests on port 80 (the URL http://msn.com is actually
sent as http://msn.com:80, the 80 being the port number).
The receiving
machine (the web server) has an application which listens on port 80. When it
sees a request on that port number, the machine knows the request should be
sent to the web server application.
On the other hand, requests on port 21 are
sent to the FTP application, and requests on port 110 go to the email POP3
application.
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