Port Checker
Test if TCP ports are open or closed on any host. Check single ports or scan common service ports.
About Port Checker
The port checker tests TCP connectivity to specific ports on a remote host. It attempts to establish a TCP connection and reports whether the port is open (accepting connections) or closed (refusing or timing out). This is useful for:
- Firewall testing - Verify that your firewall rules are working correctly
- Service verification - Confirm that a service is running and accessible
- Security auditing - Check which ports are exposed on your servers
- Troubleshooting - Diagnose connectivity issues to specific services
Understanding Network Ports
Network ports are virtual endpoints that allow multiple services to operate on a single IP address. Every TCP or UDP connection is identified by a combination of IP address and port number. Ports range from 0 to 65535, divided into three ranges: well-known ports (0-1023), registered ports (1024-49151), and dynamic/private ports (49152-65535).
When a port is "open," it means a service is actively listening for connections on that port. When it is "closed," either no service is listening, a firewall is blocking access, or the host is unreachable. Understanding which ports should be open on your servers is critical for security: unnecessary open ports increase the attack surface available to potential intruders.
Common Port Numbers
- Port 20/21 - FTP (File Transfer Protocol) - data and control
- Port 22 - SSH (Secure Shell) - encrypted remote access
- Port 25 - SMTP (Simple Mail Transfer Protocol) - email sending
- Port 53 - DNS (Domain Name System) - name resolution
- Port 80 - HTTP (HyperText Transfer Protocol) - web traffic
- Port 110 - POP3 (Post Office Protocol) - email retrieval
- Port 143 - IMAP (Internet Message Access Protocol) - email access
- Port 443 - HTTPS (HTTP Secure) - encrypted web traffic
- Port 3306 - MySQL database server
- Port 3389 - RDP (Remote Desktop Protocol) - Windows remote desktop
Frequently Asked Questions
What is a port in networking?
A port is a virtual endpoint for network communication identified by a number from 0 to 65535. Ports allow a single host to run multiple network services simultaneously. Each service listens on a specific port number so the operating system can route incoming connections to the correct application.
Why would a port be closed?
A port appears closed when no service is listening on it, when a firewall is blocking inbound connections, or when the host itself is unreachable. Closing unused ports is a security best practice to reduce the attack surface of a server.
Is port scanning legal?
Port scanning your own servers is perfectly legal and is a routine part of network administration. Scanning third-party hosts without permission can be considered unauthorized access in some jurisdictions. Always ensure you have permission before scanning hosts you do not own or manage.