Telnet doesn't have to be running on the destination server for it to be useful.
Yes. The username/password used for telnet logins are typically the same as used for ssh. So, even if telnet were disabled, the device might still be accessible, so long as the password is not changed.
just telnet client (port 23) --
You download telnet then use it like this:
telnet remoteIP remotePort
So, you can use telnet on http port (80), ssh (22), or any apps commonly installed and running on ports on the remote system.
Um, did you ever
try this?
Telnet just runs over raw TCP/IP. If you point a telnet client at a port running another protocol, it's not like it gives you a login prompt - you'll see whatever that protocol does, which most likely won't make any sense to you or be particularly useable via a telnet client.
That said, I've used a telnet client to script some simple RTSP commands against a remote serer I was testing. So, depending on the protocol, it can definitely be useful.