CONFIRMING CONNECTIVITY TO A TCP PORT WITH TELNET

While the ping command confirms connectivity, it does not necessarily mean that all TCP ports on the remote host can be reached. It is possible for a network firewall to allow or block access to certain ports on a host.

 

To check if specific TCP ports are running on the remote host, you can use the telnet command to confirm if a port is online.

# telnet destination-ip destination-port

When trying to establish a telnet connection to TCP port 80, you see an output similar to:

# telnet 192.168.1.11 80Trying 192.168.1.11…Connected to 192.168.1.11.

Escape character is ‘^]’.

In this sample output, you can see that you are connected to port 80 (http) on the server with IP address 192.168.1.11.

If you choose a port number for a service that is not running on the host, you see an output similar to:

# telnet 192.168.48.1.11Trying 192.168.1.11…

telnet: Unable to connect to remote host: Connection timed out

In this case, you can see that there is no response when you attempt to connect to port 81 on the server 192.168.48.133.

Note: Telnet is an application that operates using the TCP protocol. UDP connectivity can not be tested using Telnet.

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