Open Port Checker
Check whether a single TCP port (like 22, 80, or 443) is reachable on a host. Useful for firewall and NAT troubleshooting on your own systems.
Quick answer: Check whether a single TCP port (like 22, 80, or 443) is reachable on a host. Useful for firewall and NAT troubleshooting on your own systems.
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Frequently asked questions
- How can I see if port 443 is open?
- Enter the hostname and 443 as the port, then click Check. If a TCP handshake completes you'll see OPEN; if the host refuses the connection you'll see CLOSED; if there's no response at all the result is FILTERED (usually a firewall silently dropping the packet).
- What does "port open" vs "closed" vs "filtered" mean?
- Open: a server is actively accepting TCP connections on that port. Closed: a host is reachable but nothing is listening — it returns a TCP RST. Filtered: a firewall or middlebox dropped the packet so we got no response (timeout).
- Why does the checker say "timeout"?
- A timeout means the SYN packet got no SYN/ACK or RST back within ~3.5s. The most common causes are a firewall dropping packets, a routing issue, or the host being down.
- Can I use this for my own server?
- Yes — that's the intended use. Don't use it against systems you don't own or have permission to test.
- Does it work for IPv6?
- Yes. If the host has an AAAA record we'll prefer the v4 address for compatibility but v6 hosts are also reachable.
- What's the difference between TCP and UDP?
- TCP is connection-oriented (HTTP, SSH, SMTP) and lets us cleanly tell whether a port is open. UDP is connectionless, so a non-response can mean either 'open' or 'filtered' — that's why we only check TCP.
- Why can a port be open locally but closed externally?
- Your router/NAT or your hosting provider's firewall must explicitly forward the port to the internal machine. Without that, the service is only reachable on the local network.
- Can my ISP block ports?
- Yes. Many residential ISPs block inbound 25 (SMTP), 80, and sometimes 22 to discourage running servers on home connections.
- Is an open port always insecure?
- Not at all — every public website has 443 open. The risk is running outdated or misconfigured services on those ports. Keep software patched and use authentication.
- Which ports are most commonly used?
- 80 (HTTP), 443 (HTTPS), 22 (SSH), 25/465/587 (SMTP), 53 (DNS), 3306 (MySQL), 5432 (PostgreSQL), 6379 (Redis), 27017 (MongoDB).