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Community Member

Cisco router built-in TCP port number for telnet

Can someone pls explain to me what is the purpose of port numbers 2000, 3000, 4000? For example, I know if I use port 2097 (2000 + 97) using the example below, I can do a reverse telnet with serial3/0 interface. Are the port numbers (2000/3000/4000) user definable? When should I use 2000 or 3000 or 4000? Thank you.

# telnet a.b.c.d 2097

interface Serial3/0
physical-layer async
description Connected to TNT GHU
no ip address
encapsulation slip


line 97
modem InOut
no activation-character
transport preferred telnet
transport input telnet
transport output telnet
stopbits 1
speed 2400

Cisco#sh line
   Tty Typ     Tx/Rx    A Modem  Roty AccO AccI   Uses   Noise  Overruns   Int
     0 CTY              -    -      -    -    -      0       0     0/0       -
    97 TTY   2400/2400  - inout     -    -    -     52       3     0/0     Se3/0

Everyone's tags (2)
Hall of Fame Super Gold

Re: Cisco router built-in TCP port number for telnet

That is explained in the documentation if you take a moment to search and read it.

Community Member

Re: Cisco router built-in TCP port number for telnet

I have already searched in Cisco IOS command documentation but can't find anything useful. Still searching...

Hall of Fame Super Silver

Re: Cisco router built-in TCP port number for telnet

Hello Pllim,

line numbering is platform dependent.

for C3600

in C3600 should be from 0 to N with 32 positions for each slot so 97 and line 3/0 are a good match

in older routers it was different and aux number changed depending on router effective installed modules.

for 1800,2800, 3800

you should use 2000+line# for reverse telnet, the other port ranges 3000, 4000 have different use

I couldn't find a link at the moment but this is sure

they may be used for X.25 or LAT protocol translation

Hope to help


Community Member

Re: Cisco router built-in TCP port number for telnet

I found an example of port 4321 used for Stream TCP (see below). Is the port number range 4000 meant for Stream TCP?

Stream TCP Autocommand Procedure

In the following procedure, a line is configured so that any connection into it is automatically connected using Stream TCP to the application server at the specified IP address and TCP port (IP address and TCP port 4321 in the examples).


Community Member

Re: Cisco router built-in TCP port number for telnet

I found a previous posting. It answers my question.

Re: Reverse Telnet to Async Ports

Range           Description

---------       ------------------------------------------------------

2000            normal TELNET (full options negotiations, remote echo)

                see RFC-854, also stuff on TELNET in RFC-1123

3000            normal TELNET to rotary

4000            raw TCP (stream) - no TELNET NVT.  See RFC-793.

5000            raw TCP (stream) to rotary

6000            TELNET binary mode - see RFC-856

7000            TELNET binary mode to rotary

8000            forward Xremote

9000            reverse Xremote

10000           reverse Xremote to rotary

telnet = CR LF

binary = CR NULL          6000

stream = No negotiations     4000

Stream would usually be to a printer; binary would usually be to a host application that is running.

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