What network ports are identifiable

If you have any sort of remote access to your network or are running any sort of servers or phone system, then you will have ports open to the internet. When an outsider attempts to connect, the port may respond as open, closed or filtered

closed means the port is not listening. In technical terms, a RST is sent in response to an SYN. Open means an ACK is sent in response to the SYN. Hackers can try to identify what service is on the port. Filtered means no ACK is sent at all. There may or may not be a service behind the port

You can test what is visible with nmap. Some basic instructions are here

You can download nmap here, unzip it into a clean directory and run from a command prompt

In our case, nmap was able to identify the port and the type of vpn service that was running. Compiling a list of ip addresses, ports and services allows systematic and automated hacking