Default number 20 in line numbers of Telnet

Unanswered Question
Mar 26th, 2009

Hi guys,

telnet x.x.x.x 20yy

yy is the decimal line number to which you want to connect and 20 is default.

What's that 20 means?

May I know please?



I have this problem too.
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Collin Clark Thu, 03/26/2009 - 07:39

It's the virtual line number assigned by the device. A show line will show the virtual lines on the system.

Hope that helps.

ronmarcojr Thu, 03/26/2009 - 18:50

Yes, a show line command will show the virtual lines on the system, but once you ping the IP address it will also show you the virtual line WITH THAT NUMBER 20 inside a parenthesis. Am I right?

If it's line 34 it will say, when you're trying to ping, 2034.. Is that default number 20 (with virtual line 34) assigned by the device? Is that what you mean? Well, I still didn't get it. Apologize.

Thanks.. =)

JamesLuther Fri, 03/27/2009 - 03:02


The number 20yy (ie 2034) is the TCP port number that you're connecting to

Normally when you telnet you go to port 23 but if you put a port number on the end you'll connect to another TCP socket. ie

telnet goes to port 23

telnet 25 goes to smtp

telnet 80 goes to http


TCP ports 0-1023 are well known reserved port numbers and so cisco have just added 20 to the front of the line number in order to aviod the lower range of TCP ports.

Therefore the number 20 is just arbitary. See link below for a full list of TCP ports


Collin Clark Fri, 03/27/2009 - 06:31

When you install a card with A/S lines, new lines are installed (ie 20-27 for an 8 port card). Here's a link on the line numbering by platform.

To establish a reverse Telnet session to an asynchronous line, determine the IP address of your LAN (Ethernet) interface or loopback address, then enter a Telnet command to port 2000 + n on the access server, where n is the installed asynchronous line number. For example, to connect to the console of the router attached to line 1 (which is really asynchronous line 20)-

router# telnet 2020

Trying, 2020 ... Open


The 2000 is something decided to use and AFAIK you can't change it.

Hope that helps.


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