On 20 Apr 99, at 21:09, Mick Fitzpatrick wrote:

> Can someone please give me a brief summary of Telnet?

  Back in the Darke Ages (up to about 1992), most desktop access to larger
systems was over serial lines using "dumb" terminals (not much more than
screen and keyboard).  All "intelligence" was on the host end.  Most
terminals supported a few special command codes for moving the cursor around,
and there were programs for PCs and Macs and so on that recognized those
codes and so let you use a desktop computer as a terminal.  Dial-in consisted
of routing a serial terminal connection over a modem link; configuring the
modem was easier if you let a program do it.
  While some ISPs still let users dial in to a "shell account", most dial-in
access these days is over SLIP or PPP, serial emulation of a point-to-point
network link.  Instead of keystrokes and text, what goes back and forth on
the wire are network packets, and only your local machine sees actual
keystrokes.

  Telnet is a protocol, layered on top of TCP/IP, that lets software designed
to work with locally-attached serial terminals be used from remote network-
connected machines.


  Old style:  Host runs an application that talks to local serial terminals.
User runs a "terminal emulator" program on his PC, connected directly or over
modem to a serial port on the host.

  New style:  Host runs an application that talks to local serial terminals,
and a "telnet server" which accepts incoming network connections and
translates them to look like serial terminal connections.  User has a network
connection (wired or over modem) from his PC, and runs a "telnet client"
which provides a window in which to type and see text, and translates that to
a protocol for connection to the "telnet server" on some (specified) remote
machine.


  It turns out to be more useful than it sounds.  Many of the other kinds of
server programs used on the internet -- POP, SMTP, and so on -- use protocols
which *can* be used interactively with text.  [This helps make them easy to
test/debug.]  So although it's not always a good idea, you can often use a
telnet client to connect to other kinds of server programs besides telnet
servers.
  Under TCP/IP, server programs accept incoming connections on some "port
number", and so you can talk to different server programs on the same host by
specifying different port numbers.  If there is a telnet server, it's
normally on port 23; you can easily find port numbers for other standard
servers listed on various web pages.


David G

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