A show run telnet will display any telnet-related commands in the running config.
ciscoasa(config)# sh ru telnet telnet timeout 5
By default, the ASA only has a single command preconfigured: a telnet session timeout for the default 5 minutes. The available range is 1 to 60 minutes. To change the session timeout, use the telnet timeout command. For example, to change the timeout to 60 minutes:
ciscoasa(config)# telnet timeout 60
To specify the hosts allowed to telnet into the ASA, use the telnet command to specify the IP address (or range) and the interface to which the host(s) will connect.
To permit a single host at 184.108.40.206 to telnet into the inside interface, use a mask of 255.255.255.255:
ciscoasa(config)# telnet 220.127.116.11 255.255.255.255 inside
To permit all hosts in the 192.168.3.0 subnet to telnet into the inside interface, include the subnet mask:
ciscoasa(config)# telnet 192.168.3.0 255.255.255.0 inside
To permit all hosts to telnet into the inside interface, use the 0 0 shorthand:
ciscoasa(config)# telnet 0 0 inside
To view all current telnet sessions, use the who command:
ciscoasa(config)# who 0: 18.104.22.168
The number before the colon is the session ID. In the example above, the host at 22.214.171.124 is telnetted into the ASA with a session ID of 0.
Now if I do a no telnet 126.96.36.199 255.255.255.255 inside, this will remove 188.8.131.52 from the list of hosts permitted to telnet into the ASA. However, the command will not terminate an existing telnet connection from that source IP address. It will only prevent new connections from 184.108.40.206. To terminate an existing session, I have to run a kill command and specify the session ID that I want to terminate.
ciscoasa(config)# who 0: 220.127.116.11 ciscoasa(config)# kill 0