CONNECTION ESTABLISHMENT

   The TELNET TCP connection is established between the user's port U
   and the server's port L.  The server listens on its well known port L
   for such connections.  Since a TCP connection is full duplex and
   identified by the pair of ports, the server can engage in many
   simultaneous connections involving its port L and different user
   ports U.

   Port Assignment

      When used for remote user access to service hosts (i.e., remote
      terminal access) this protocol is assigned server port 23
      (27 octal).  That is L=23.