Subj : Echo Traffic
To   : Richard Town
From : Stewart Honsberger
Date : Thu Mar 21 2002 04:19 pm

Hello Richard!

Mar 19 2002 17:08, Richard Town wrote to Ben Carpenter:

 RT> So your analagy means Telnet is always on door #23, and HTTP on door #80, 
 RT> etc?  If so, how come it's configurable?  What's the convention?

No, it doesn't mean 'always' - you can do pretty much whatever you like. There 
are 65 thousand ports to choose from, but don't expect many client applications 
to work if you go and move all your services to random ports just 'because'.

Telnet is /expected/ to be on port 23, HTTP is /expected/ to be on port 80, ftp 
is /expected/ to be on port 21.

Imagine, if you will, that you enter an appartment building. Your friend tells 
you that he lives in the fifth appartment of the third floor. So you punch 
"305" on the buzzer panel and get the janitor, or perhaps an ornery old woman. 
Isn't that inconvenient?

Client applications like web browsers are designed to look, by default, for web 
pages on TCP port 80. When you enter a URL, you only have to specify the URL 
without having to bother with the port. Very convenient.

If I put my webserver on port 81 just to be different, how would anybody be 
able to hit my website? I'd have to tell them all about it, that's how. Imagine 
what a terrible burden it is to try to get everybody to remember that when they 
want to go to /MY/ website they must remember a completely different port 
number. Now, of course, your web site will ALSO be on a different port. Maybe 
Yahoo! decides that they prefer port 321, so that's three sites people have to 
memorize.

Eventually at this rate, the WWW would collapse out of frustration.

Mail servers are unattended beings. They expect to be able to communicate with 
other mail servers on port 25. If it can't connect to the mailserver because 
it's now on port 2525, the mail simply won't get to its destination. You just 
can't tell every mailserver in the world to alter their system configuration to 
accomodate your desire to not 'do it because everyone else does it'.

Get it?

 RT> I can't go on just doing that coz everyone else does it!

Actually, that's how the Internet works. People agree on a set of standards and 
then proceed to meet them. Otherwise the world would be perpetually 
unconnected.

It's cooperation, not conformity.

 RT> Not that the textbooks are much help either.  New Riders MCSE for 
 RT> 70-068 only talks about comms ports, and the earlier covering 70-58 
 RT> doesn't even have Port listed in its index!

When you're bored, hit http://blackdeath.2y.net/services.txt

That's a copy of the /etc/services file that comes with (most) Linux 
distributions. It lists just about every service under the sun and its 
corresponding port, sorted by port number starting with 1.

Enjoy it in good health.

[Stewart Honsberger] [blackdeath@softhome.net] [http://blackdeath.2y.net/]

 "But you've got to hand it to IBM, they know how to design
 hardware. The servers all had handles to pick them up and throw
 them out of the window...."  -- Juergen Nieveler in the Monastery

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