What's the difference between a closed port and a filtered port?
A closed port indicates that no application or service is not listening for connections on that port. A closed port can open up at any time if an application or service is started.
A filter port indicates that a firewall, filter, or other network issue is blocking the port. Some standard services that can create a filter port can be, but not limited to, a server or network firewall, router, or security device.
A common tool that is used to check the status of ports is Nmap. You can find more infomration about Nmap from their website.