Telnet Port

[kapilbajpai88]

Hi All,

I am new to this forum, and have a qury regarding port number. I am using a cisco router(don't know model number) at our workplace and need to know the port number that is used by telnet application. I am getting error that Telnet can't make session as port:23 is not there...but when I checked NETSTAT, I didn't get any port number like 23. Infact, I amnot sure that port 23 is at all in use.

Could anyone can help me checking which port is being used by Telnet as default, and how can we check that ??

Many thanks in advance,

Kapil.

[Joe Clarke]

The telnet port is tcp/23. There is no netstat application on IOS, but if you have "transport input all" or "transport input telnet" under your line vty configuration, the device is listening on tcp/23. There could be an access-list blocking telnet still, however.

On your client, unless it has a telnet daemon running, you will not see tcp/23 in use in netstat. The telnet client on your machine will open a socket to tcp/23 on the IOS device, but use a high (ephemeral) TCP port on the client. Netstat will show this socket as being established with the remote end using tcp/23, and the local end using this high TCP port.

[kapilbajpai88]

hi jclarke,

Thank you for the prompt reply....

But I am novice with these technical jargons and can't get much out of it. I googled this in the meantime, and get that port:23 is default for telnet application. But I think that port :23 should not be affected by the router at all...right. Can firewall bethe issue for this , as I am using an internal firewall over the PC ?

Could you please suggest me how can one open or close any port over a PC? What is the actual process for thsi ?

Thank You,

Kapil.

[Joe Clarke]

Modern versions of Windows ship with a firewall. If you enable this, and add a rule to block tcp/23, then that will block the port. I don't use Windows, so I don't have the exact process for this.

However, from a Windows machine, if you telnet to an IOS device, that will attempt to open a socket to port 23 on the IOS device. If that fails, then the IOS device is either filtering tcp/23, or has an access-list configured on it, blocking telnet on the VTY lines. You would see this in the IOS config as an access-class configured under the line vty section towards the bottom of the config.

[kapilbajpai88]

Hi jclarke,

Thank you again for the response..

But this is getting even more confusing for me now. I am using a win-xp(32-bit) PC within LAN, and getting such an error then how can I troubleshoot this ?

Thank You,

Kapil

[Joe Clarke]

What is the exact error you're getting in XP? Post the output of "show running" from the IOS device. Note: you'll need a console connection to the IOS device if you cannot telnet to it.