Telnet is a protocol that you can use to connect remotely to network devices (such as Cisco IOS Routers) so you can configure/monitor them. On Cisco IOS you can use Telnet, but it is advisable that you use SSH instead because it is more secure that it sent everything in an encrypted way, while with Telnet everything is sent as plain-text. I do have a lesson speaking about SSH and how to configure it on Cisco IOS, you can check it here.

I have the following scenario:

My laptop is connected to F0/0 interface of the Cisco Router, and they have the ip addresses as show in the graph. For Telnet to work, we should be able to reach to the Cisco Router (whether from inside or from outside via the internet/VPN). Let's ping from the PC to R1 IP address.

Pinging with 32 bytes of data:
Reply from bytes=32 time=5ms TTL=255
Reply from bytes=32 time=9ms TTL=255
Reply from bytes=32 time=11ms TTL=255
Reply from bytes=32 time=11ms TTL=255
Ping statistics for
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 5ms, Maximum = 11ms, Average = 8ms

As you can see, we do have reachability to the Cisco Router.

Let's start checking on R1 how many lines are available for VTY.

R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#line vty 0 ?
  <1-903>  Last Line number
R1(config)#line vty 0

WoW. 904 lines are available (including the 0). What does this mean? This means that 904 simultaneous telnet connect can happen to the Cisco router. I know you may be thinking that this is a lot, but on small Cisco equipment you may see from 0 to 4, so that means 5 concurrent telnet sessions can happen.

Let's enable now the telnet and put a password for it.

R1(config)#line vty 0 4
R1(config-line)#password 123456

I have enabled Telnet for 5 lines only, put a password of "123456" (plus use a strongest password always) and at the end I put the command "login" to enable the Telnet.

Now everything looks good. Let's open putty software and do telnet to the Cisco Router.

Telnet works on port 23 by default, so you have to put the router IP address and port 23 then click on open.

Once done, it will ask for the password.

Let's put the password in.

Here we go. We could log in into the Cisco Router via Telnet. The password is always invisible on Cisco Router, but I have already entered it and you see I am on the user mode on R1.

Now if you open 5 telnet sessions from your PC to the Cisco Router, then will all work. If you open a 6th session, you will see it is not going to work. Why? Because we have limited the number of Telnet sessions to be 5 from we said "line vty 0 4".

Excellent!!!! So this is how you can configure Telnet on Cisco Router. If you like this lesson, please share it to your friends/colleagues and if you have any question(s) please leave it in the comment below and I will get back to you ASAP.

See you in another lessons.

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