We need to check if things are working at the transport layer. For this, there are two super powerful tools at your disposal. Netcat on Linux and MacOS and Test-NetConnection on Windows.
The Netcat tool can be run through the command nc and has two mandatory arguments, a host and a port. Running nc google.com 80 would try to establish a connection on port 80 to google.com. If the connection fails, the command will exit. If it succeeds, you’ll see a blinking cursor waiting for more input. This is a way for you to actually send application layer data to the listening service from your own keyboard. If you’re only curious about the status of a port, you can issue the command with the -z flat, which stands for zero input output mode. The -v flag, which stands for verbose basically means talking too much.
So the -z and -v flags tell you if connection to the porting is possible or not.
On, windows we use Test-NetConnection.
Default will use the ICMP echo request like ping, but it will display way more data, including the data link layer protocol being used. When you issue Test-NetConneciton with the dash port flag, you can ask it to test connectivity to a specific port. It’s important to call out that both Net cat and Test-NetConnection are way more powerful than the brief port connectivity examples we’ve covered here.