Port Checker

Check whether a TCP port is reachable on a host. Enter a target to get the exact nmap and telnet commands to test common ports, plus a reference of what each well-known port is for.

Port Checker
Enter a host above to get ready-to-run port-check commands.

What the Port Checker does

A port checker tests whether a specific TCP port on a host is open, closed, or filtered. Ports are the numbered endpoints services listen on — 80 for HTTP, 443 for HTTPS, 22 for SSH — and knowing which are reachable is essential for troubleshooting connectivity and reviewing exposure.

Because a browser cannot open raw TCP sockets for security reasons, live scanning must be done from your own machine. This tool gives you the precise nmap and telnet commands for your target, along with a guide to reading the results and a reference of the ports that matter most.

How to read a port's state

  • open — a service is actively listening and accepting connections on that port.
  • closed — the host responded but nothing is listening on that port.
  • filtered — a firewall is silently dropping probes, so the state can't be confirmed.
  • Common ports: 22 SSH · 25 SMTP · 53 DNS · 80 HTTP · 443 HTTPS · 3306 MySQL · 3389 RDP.

Worked example

Input: example.com

nmap -p 80,443,22,21,25 example.com
telnet example.com 80

Run the nmap command to scan several ports at once, or use telnet for a quick single-port check — a blank screen means the port is open, while an immediate refusal means it's closed.

Common use cases

  • Confirm a web, SSH or database server is actually listening on its port.
  • Diagnose whether a firewall is blocking a connection.
  • Verify port-forwarding rules on your router are working.
  • Audit which services are exposed on a host you administer.

Port Checker — Frequently Asked Questions

Why can't a website scan ports directly in my browser?▾

Browsers deliberately block raw TCP socket access as a security measure, so no web page can port-scan an arbitrary host from your machine. Reliable checks must be run from a local tool such as nmap, telnet, nc (netcat) or PowerShell's Test-NetConnection.

What does a 'filtered' port mean?▾

Filtered means a firewall or packet filter is silently dropping the probe, so the scanner cannot tell whether a service is listening behind it. It differs from 'closed', where the host actively replies that nothing is on that port.

Which ports should I check first when troubleshooting?▾

Start with the port for the service in question — 80/443 for web, 22 for SSH, 3389 for RDP, 3306 for MySQL — then widen out. Scanning only the relevant ports is faster and less likely to trip intrusion-detection systems.

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