01What is a port check for?
A port is a numbered door on your connection: each network service listens on one (443 for HTTPS, 22 for SSH…). This test attempts a TCP connection from the Internet to your own IP address and tells you whether the door answers. It is the “outside” view, the one a remote server or peer has — different from what you see on your local network.
The most common use: verifying a port forward (NAT) on your router after setting up a game server, a camera, SSH access or a self-hosted web server. If the test says “closed” while a service is running, the block almost always comes from the router or a firewall. For safety, this test can only target your own IP (shown above), never someone else's.
02The most common ports
- 22 — SSH
- Remote command-line administration. Only expose it with keys and, ideally, a non-standard port.
- 80 / 443 — HTTP / HTTPS
- Web servers. 443 (encrypted) is the norm; 80 mostly redirects to 443.
- 21 — FTP
- Legacy, unencrypted file transfer. Prefer SFTP (over the SSH port) today.
- 25 — SMTP
- Email between servers. Often blocked outbound by consumer ISPs to fight spam.
- 3389 — RDP
- Windows Remote Desktop. A frequent attack target: never expose it directly without a VPN.
- 3306 — MySQL
- Database. Should never be open to the Internet: keep it on the LAN or behind a tunnel.
- 8080 — HTTP alternate
- Often an application server or a development proxy.
03From the command line
The same URL answers in plain text for curl, and the JSON API returns the structured state. The port tested is always the one of the requesting IP:
$ curl "its-my-ip.com/port?port=443" 203.0.113.42:443 open (34ms) $ curl "its-my-ip.com/api/port?port=443" { "ip": "203.0.113.42", "port": 443, "state": "open", … }
04Frequently asked questions
Why can I only test my own IP address?
Testing ports on a machine you don't own is port scanning, an intrusive and often illegal activity without authorization. This tool therefore deliberately limits itself to the IP address you are visiting from — the only legitimate, risk-free use case: diagnosing your own connection.
The test says “closed” but my service is running, why?
In order of likelihood: no port forward (NAT) configured on your router, a firewall (router, system or cloud provider) blocking it, a service listening only on 127.0.0.1 instead of 0.0.0.0, or carrier-grade NAT (common on cellular and some fiber ISPs) making your IP unreachable from outside.
What is the difference between a closed and a filtered port?
Closed: the machine actively answers that nothing is listening (the system returns a refusal). Filtered: a firewall silently drops the request, with no answer. From the outside, both appear as a connection that doesn't succeed; this test therefore groups them as “closed or filtered”.
Is opening a port dangerous?
Every open port is a potential attack surface. That's fine if the service behind it is up to date, authenticated and needed. The risk comes from ports opened by mistake to vulnerable or password-less services. Rule of thumb: only open what you need, and prefer a VPN for administration (SSH, RDP).
Why does the test fail over IPv6?
The test targets the IP address of your connection to this page. If you arrive over IPv6, it is a port on your IPv6 address that is tested — but many home services are only reachable over IPv4 behind NAT. Force IPv4 in your browser, or test from an IPv4 connection, to validate a classic NAT forward.