Telnet is a virtual terminal protocol used to log into a computer across the Internet or an intranet. RFC 854 contains the Telnet protocol specification, and RFC 855 contains the Telnet options specification. Telnet connects to a remote host by establishing a TCP connection over port 23 and passing keystrokes from the user's keyboard to the remote machine, as if they had been entered on a keyboard attached to the remote host. The Telnet protocol uses TCP for transport. No modifications have to be made to Telnet, because Telnet does not embed addresses in its protocol.
I had a Telnet server running on Windows, accessing it from the Linux host Ford, and also did the oppositerunning an IPv6-enabled Telnet server on the Linux host and accessing it with the Microsoft Telnet client. Both ways are no problem. The Telnet protocol works over TCP over IPv6 with the standard Telnet port number 23. Figure 9-10 shows the Telnet session.
Figure 9-10. A Telnet session over IPv6
The figure shows the negotiation of the Telnet session. In the detail window, you can see the layers: the MAC layer with Ethertype 86DD, the IPv6 layer using TCP value 6 in the Next Header field, the TCP layer using port 23 for Telnet, and the Telnet header. Telnet also sends passwords and all session data in clear text that an intermediary may discover. Software such as SSH provides an encrypted terminal for security. SSH is IPv6-ready today and available on many platforms.