**nmap** is a useful tool to verify a server is up and the applications are listening for connections. Here are some common troubleshooting scenarios. From an outside workstation, scan your server's IP address `nmap`. These are useful command switches: **-Pn** : Treat all hosts as online and skip host discovery. **-p** : List of ports to scan. **--reason** : Display the reason a port is in a particular state. Example: To scan ports 22, 53, 80, and 443 on IP address 192.0.2.2: ......