Port Checker

How port checking works

A network port is a virtual endpoint that identifies a specific process or service running on a computer connected to a network. Ports range from 0 to 65535, with the first 1024 ports reserved for well-known services. Port 80 is reserved for HTTP traffic, port 443 for HTTPS, port 22 for SSH, port 21 for FTP, and port 25 for SMTP email delivery. When a client wants to connect to a service, it must know both the IP address of the host and the port number on which the service listens.

In a browser environment, the only protocol available for network connections is HTTP/HTTPS via the fetch API. When you check port 443 on a hostname, the tool constructs a URL like https://hostname:443/ and attempts to fetch it. If the server responds, even with an error status like 403 or 404, the port is confirmed open. A network error (connection refused, timeout, or DNS failure) means the port is unreachable from your location. For HTTPS, the browser also validates the TLS certificate, so a certificate error can also cause a failure even if the port is open.

Ports 22 (SSH), 21 (FTP), and 25 (SMTP) cannot be tested from the browser because they use protocols other than HTTP. JavaScript's fetch API can only make HTTP and HTTPS requests. Testing these ports requires a server-side component that opens raw TCP sockets and checks for a connection acceptance or timeout. The tool displays "not testable" for these ports and explains the limitation. For a complete port check on any port, you would need a desktop tool like Nmap or a web service backed by a TCP socket server.

Understanding port states

Network ports can be in one of three states from the perspective of a remote client: open, closed, or filtered. An open port means an application is actively listening for connections on that port. The operating system completes the TCP three-way handshake and passes the connection to the listening application. For HTTP ports, an open port typically means a web server is running. If you receive an HTTP response, even a 403 Forbidden or 404 Not Found, the port is confirmed open.

A closed port means no application is listening on that port. The operating system responds to the initial SYN packet with a TCP RST (reset) packet, immediately refusing the connection. The browser translates this into a "connection refused" error. A filtered port means the packet was silently dropped, typically by a firewall rule or network ACL. No response is sent to the SYN packet, and the client waits until a timeout occurs. Filtered ports are common for internal services that should not be accessible from the public internet.

When to use a port checker

A port checker is useful when you need to verify that a service is running and reachable from your network location. Web developers deploying a new server need to confirm that port 80 and 443 are open and the web server responds. System administrators migrating a service to a new host can quickly verify that the correct ports are open on the new server before directing production traffic to it. Security professionals use port checks as a basic reconnaissance step to verify firewall rules are working as intended.

For home users, a port checker helps diagnose whether their ISP blocks certain ports (some ISPs block port 25 to prevent spam, for example) or whether their router port forwarding rules are correctly configured. When setting up a home server, game server, or remote access solution, checking port reachability from an external network confirms that the router is properly forwarding traffic to the internal server.

Frequently asked questions