What's the difference between a closed port and a filtered port?
A closed port means that no application or service is not listening for connections on the port number at that time. A closed port can open up at any time if an application or service starts to listen on the port.
A filter port means that a firewall, filter, or other network obstacle blocks the port so that connections cannot tell whether it is open or closed. Some standard services that can create a filter port can be, but not limited to, a server firewall, network firewall, or other filters to restrict access.
A common tool that is used to check the availability of ports is Nmap. You can find more infomration about Nmap from their website.