Port checker

Port status

open: TCP connections are being accepted on that port, or there are some UDP or SCTP associations on it.
closed: There is no application listening on the port.
filtered: The packets are sent to the port, but a packet filtering (e.g., firewall) prevents them from reaching it.
unfiltered: That port is accessible, but unable to determine whether it is open or closed.
open | filtered: Unable to determine whether the port is open or filtered.
closed | filtered: Unable to determine whether the port is closed or filtered.

The most common ports

20-File Transfer Protocol (FTP) Data Transfer
21-File Transfer Protocol (FTP) Command Control
22-Secure Shell (SSH)
23-Telnet-Remote login service, unencrypted text messages
25-Simple Mail Transfer Protocol (SMTP) E-mail Routing
53-Domain Name System (DNS) service
80-Hypertext Transfer Protocol (HTTP) used in World Wide Web
110-Post Office Protocol (POP3) used by email clients to get email from a server
119-Network News Transfer Protocol (NNTP)
123-Network Time Protocol (NTP)
143-Internet Message Access Protocol (IMAP) Management of Digital Mail
161-Simple Network Management Protocol (SNMP)
194-Internet Relay Chat (IRC)
443-HTTP Secure (HTTPS) HTTP over TLS/SSL

• Analyzing the cause when you have trouble with the connection to applications.
• Checking the state of a port to avoid accepting the external attacks to the website.