What Is a Port Checker?
A port checker tests whether specific TCP ports on a server or IP address are accepting connections. Every networked service listens on a port number — web servers on port 80 (HTTP) and 443 (HTTPS), email servers on port 25 (SMTP), and so on. Checking ports helps verify that services are running and accessible, or that a firewall is correctly blocking unwanted traffic.
Port checking is a fundamental network diagnostic technique used by system administrators, security professionals, and developers alike. Whether you are deploying a new service, troubleshooting connectivity issues, or auditing your infrastructure for security vulnerabilities, a port checker gives you immediate visibility into which services are reachable from the outside world.
Common TCP Ports and Their Services
| Port | Service | Description |
|---|---|---|
| 21 | FTP | File Transfer Protocol — used for transferring files |
| 22 | SSH | Secure Shell — encrypted remote access |
| 25 | SMTP | Simple Mail Transfer Protocol — sending email |
| 53 | DNS | Domain Name System — resolving domain names |
| 80 | HTTP | Hypertext Transfer Protocol — unencrypted web traffic |
| 443 | HTTPS | HTTP Secure — encrypted web traffic |
| 3306 | MySQL | MySQL database server |
| 3389 | RDP | Remote Desktop Protocol — Windows remote access |
| 5432 | PostgreSQL | PostgreSQL database server |
| 8080 | HTTP-Alt | Alternative HTTP port — often used for development |
Understanding Port States
When you scan a port, it will be reported in one of three states:
- Open: The port is actively accepting TCP connections. A service is running and reachable.
- Closed: The host is reachable but no service is listening on that port. The operating system actively refuses the connection (TCP RST).
- Filtered: No response was received. A firewall, router, or security device is silently dropping the packets. This is the most secure state for ports you don't want exposed.
Learn More
Read our in-depth guide to understand the concepts behind this tool:
- How to Check Open Ports — What ports are, common port numbers, firewalls, and port forwarding explained.
Frequently Asked Questions
Is port scanning legal?
Scanning your own servers is always legal and a routine security practice. Scanning servers you don't own is a legal gray area in many jurisdictions — always get explicit permission before scanning infrastructure you don't control. This tool is designed for checking your own servers and services.
Why does a port show as “filtered”?
A filtered port means no response was received within the timeout period. This typically indicates a firewall is silently dropping packets rather than responding. From a security standpoint, filtered is better than closed because it gives potential attackers less information about your system.
Should I close all unused ports?
Yes. Every open port is a potential attack surface. Best practice is to follow the principle of least privilege — only keep ports open that are required for your services to function. Use a firewall to block everything else.
Why do results differ from my local port scan?
This tool scans from our server, not from your network. Ports that are open internally may be blocked by your router's NAT or your ISP's firewall. Additionally, some services only listen on localhost (127.0.0.1) and are not accessible from external networks.