What are the differences in closed, open, and filtered ports on a firewall?

By | 2013/08/19

Using nmap or other tools, you may have seen results that say open, closed, or filtered. Here is some insight into what this all means!

Closed Port

Send a SYN packet -> get RST packet.

Open Port

Send a SYN packet -> get SYN/ACK.

Filtered Port

If the packet is discarded and there is no response this is typically considered a filtered port.

nmap actually has a total of six different states it can record though the above three are the most common across all networking tools.