Checking for open ports

Sometimes you can't access a website, or remote service and want to check whether it's a network problem or something at the server end.

Nmap is the most common way of doing that and is usually one of the first things installed in Linux. If not you can install it with (Ubuntu/Debian-based distributions):

sudo apt install nmap

Then you can check for open ports with:

nmap -p <port> <host>

If you're working in Windows and can't be bothered installing Linux Subsystem for Windows, or the full Nmap for Windows app, you can use powershell for the same thing:

Test-NetConnection -Port <port> <host>