Port Checker
Test whether a specific TCP port is open or closed on any host. Useful for firewall troubleshooting and service verification.
What is a TCP port?
A TCP port is a numbered endpoint on a networked device (0-65535). Different services listen on different ports: web servers use 80 (HTTP) and 443 (HTTPS), SSH uses 22, MySQL uses 3306. A port checker tests whether a TCP connection can be established to a given host on a given port.
Open vs closed ports
Open means a TCP connection was successfully established — something is actively listening. Closed can mean the service is not running, a firewall is blocking the connection, or the host is offline. This tool tests from external servers, so it reflects public internet reachability.
Common ports reference
Port 22: SSH. Port 25: SMTP. Port 80: HTTP. Port 443: HTTPS. Port 3306: MySQL. Port 5432: PostgreSQL. Port 6379: Redis. Port 3389: RDP. Ports 8080/8443: HTTP/HTTPS alternatives. If a port shows closed unexpectedly, check firewall/security group rules first.
Firewalls and port filtering
Cloud providers block all inbound traffic by default — you must explicitly allow specific ports in security group rules. Linux servers use iptables or ufw. Run 'ufw status' or check your cloud console's security group inbound rules when a port appears closed unexpectedly.