The utility telnet gets a bad wrap sometimes but it is a powerful troubleshooting tool.

You can use it to test if any port is listening – aka “open” – on any host.

There’s a HUGE ASSUMPTION we are making when using this tool, but we’ll get to that in a moment.

Here’s how to confirm if port 80 is open on host google.com

telnet google.com 80

And here’s my demo of telnet:

Demo 1:

Demo 2:

Now, about that HUGE ASSUMPTION mentioned earlier..

We are using telnet to test end-to-end connectivity of:

1. The client you are running the command on
2. To the server you are connecting to

We have zero visibility of the network infrastructure itself.

If you cannot connect to a port on a remote host, this does not necessarily mean the port and/or host is down.

It could be network problem. The switches and/or routers could be having an issue.

But assuming the network is up, is an assumption you should always start with..why?

It allows you to do your due diligence and rule out everything you can server-side.

After you’ve ruled out all server-side issues; then you can engage the network team about looking into the network infrastructure itself.