closed Port in nmap

The six port states recognized by Nmap

       open   An application is actively accepting TCP connections or UDP packets on this port. Finding these
              is often the primary goal of port scanning. Security-minded people know that each open port is
              an avenue for attack. Attackers and pen-testers want to exploit the open ports, while
              administrators try to close or protect them with firewalls without thwarting legitimate users.
              Open ports are also interesting for non-security scans because they show services available for
              use on the network.

       closed A closed port is accessible (it receives and responds to Nmap probe packets), but there is no
              application listening on it. They can be helpful in showing that a host is up on an IP address
              (host discovery, or ping scanning), and as part of OS detection. Because closed ports are
              reachable, it may be worth scanning later in case some open up. Administrators may want to
              consider blocking such ports with a firewall. Then they would appear in the filtered state,
              discussed next.

       filtered
              Nmap cannot determine whether the port is open because packet filtering prevents its probes from
              reaching the port. The filtering could be from a dedicated firewall device, router rules, or
              host-based firewall software. These ports frustrate attackers because they provide so little
              information. Sometimes they respond with ICMP error messages such as type 3 code 13 (destination
              unreachable: communication administratively prohibited), but filters that simply drop probes
              without responding are far more common. This forces Nmap to retry several times just in case the
              probe was dropped due to network congestion rather than filtering. This slows down the scan
              dramatically.

       unfiltered
              The unfiltered state means that a port is accessible, but Nmap is unable to determine whether it
              is open or closed. Only the ACK scan, which is used to map firewall rulesets, classifies ports
              into this state. Scanning unfiltered ports with other scan types such as Window scan, SYN scan,
              or FIN scan, may help resolve whether the port is open.
       open|filtered
              Nmap places ports in this state when it is unable to determine whether a port is open or
              filtered. This occurs for scan types in which open ports give no response. The lack of response
              could also mean that a packet filter dropped the probe or any response it elicited. So Nmap does
              not know for sure whether the port is open or being filtered. The UDP, IP Protocol, FIN, Null,
              and Xmas scans classify ports this way.

       closed|filtered
              This state is used when Nmap is unable to determine whether a port is closed or filtered. It is
              only used for the IPID Idle scan.