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From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.ham-radio.packet
Subject: Re: TCP Programmers Guide
Message-ID: <756@petrus.UUCP>
Date: Fri, 13-Dec-85 22:40:07 EST
Article-I.D.: petrus.756
Posted: Fri Dec 13 22:40:07 1985
Date-Received: Thu, 19-Dec-85 04:17:56 EST
References: <751@petrus.UUCP> <823@amdimage.UUCP>
Organization: Bell Communications Research, Inc
Lines: 30

Relax. "Well known port numbers" are needed only for SERVICE TYPES that are
likely to become widespread on the Internet. When establishing a connection
as a client, you simply pick a port number that is unused on your machine
(convention holds that you use one between 1000-65535). If you want to
experiment with a new service, you are free to pick a number at random for
testing, and no one could care less.

The reason there are "well known port numbers" and a procedure for
registering them is to simplify matters should your service become
widespread on the network. Using a standard port number allows you to avoid
having the same service appearing on a different port number on each machine
on the Internet.  For example, if I want a login prompt from ucbvax over the
Internet, I initiate a TCP connection to ucbvax port 23, because 23 is the
"well-known port number" for the Telnet (remote login) service. I use the
same number with any other machine on the net should I want a login prompt
from it; I don't need a database per machine to tell me where to find each
service.

The only situation where we might want a formal port number assignment would
be if we want to set up a new service which is likely to become widespread.
For example, someone might want to put a UNIX system up on the air with a
server process that provides a W0RLI-style PBBS and make it available to
people without forcing them to go through the normal UNIX-style login
procedure. I could establish and register a well-known port number for
"W0RLI PBBS Service", so that any other systems that offer the same service
could use a the same, standard number. You connect to this special port and
you get the bulletin board; you want to login to "real" UNIX, you connect to
port 23.  It's just an operational convenience, that's all.

Phil