Port Checker — Test Open and Closed Ports Online
Test if any TCP port is open or closed on any host or IP address. Free online port checker for developers and network admins. No software required. Instant results.
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Launch the Port Checker tool — fully free, no signup required.
What Is a Network Port?
A network port is a numbered logical channel on a host. The IP address gets traffic to the right machine; the port number gets it to the right program on that machine. Ports range from 0–65535. The first 1024 are reserved "well-known" ports (HTTP on 80, HTTPS on 443, SSH on 22). Higher numbers are used for custom services, games, and apps.
Common Port Numbers Reference
These are the ports you'll see most often when checking services, opening firewall rules, or forwarding traffic on a home router. If a service is misbehaving, confirming the right port is open is usually the fastest first diagnosis.
| Port | Protocol | Common Use |
|---|---|---|
| 21 | FTP | File transfer |
| 22 | SSH | Secure remote access |
| 25 | SMTP | Email sending |
| 53 | DNS | Domain name resolution |
| 80 | HTTP | Web traffic |
| 110 | POP3 | Email retrieval |
| 143 | IMAP | Email access |
| 443 | HTTPS | Secure web traffic |
| 3306 | MySQL | Database connections |
| 3389 | RDP | Remote desktop |
| 8080 | HTTP Alt | Web development |
| 27017 | MongoDB | Database connections |
How to Use the Port Checker
Enter the host as either an IP address (1.2.3.4) or a domain (example.com), then the port number (1–65535), and run the check. The tool opens a TCP connection from our server to that endpoint and reports whether it succeeds, fails, or times out — same as a real client elsewhere on the internet.
Why Is My Port Closed?
The four most common causes are: a firewall (Windows Defender, ufw, cloud security group) blocking inbound traffic; the service simply not running on that host; the router not forwarding the port to the right internal device; and the ISP blocking the port at the network edge (commonly 25, 80, and 443 on residential plans). Check each in that order.
What It Is
A Port Checker is a tool that tells you whether a specific TCP port on a publicly reachable host is open, closed, or silently filtered. Network ports are the numbered channels that let multiple services share a single IP address: web traffic on 80 and 443, mail on 25/465/587/993, SSH on 22, databases on 3306/5432/27017, game servers on whatever the developer chose. When something on the network isn't working — a self-hosted app you can't reach, a game server friends can't join, a webhook endpoint a third party can't call — the first question is almost always 'is the port actually open from the outside?' The DigiMetrics Port Checker answers that question by running the probe from our server, so the result reflects the public internet's view rather than your local network's.
How It Works
The check opens a TCP three-way handshake from our backend to the host:port you specify. If the remote service replies with SYN-ACK, the port is open. If the remote host replies with RST, the port is closed (the host is alive, but no service is bound). If the probe receives no reply at all within the timeout, the port is filtered — a firewall, router ACL, or cloud security group silently dropped the SYN. Each of these three states points at a different root cause, which is why the tool surfaces them separately rather than collapsing 'closed' and 'filtered' into a single 'not open'.
Real-World Examples
A homelab user trying to expose Nextcloud at port 443 runs the check, sees 'filtered', and realizes their ISP blocks inbound 443 on residential plans. A game-server admin opens TCP 25565 on the router but the check still reports closed — the Minecraft server isn't bound to 0.0.0.0, only to 127.0.0.1. A developer setting up a webhook receiver confirms port 8080 is reachable before sharing the URL with a partner. A DevOps engineer validating a new EC2 instance verifies that only port 22 is open and 3306 (MySQL) correctly isn't. A security auditor confirms that a customer's printer isn't exposing port 9100 (raw print) to the internet — a surprisingly common finding.
Common Mistakes to Avoid
Don't test private IPs (10.x, 172.16–31.x, 192.168.x) — they're not routable on the public internet, so the result will always be filtered. Don't test localhost — it's only meaningful from the machine itself, not from our server. Don't scan ports on systems you don't own and have no permission to test; in most jurisdictions that crosses into computer-misuse territory. Don't assume an open port means a healthy service — open just means the TCP layer accepts connections; the service on top might still be broken. And don't test ports on shared cloud hosting where your provider explicitly blocks inbound traffic — the result reflects their policy, not your config.
Security Implications
Every open port is potential attack surface. Best practice is to expose only what you need (HTTPS for web, 22 for SSH, maybe a VPN endpoint) and to block everything else at the perimeter. Use the Port Checker as a post-deployment sanity check: after every config change, re-scan and verify that no surprise ports were opened. Critical ports to keep firewalled from the public internet include 3306/MySQL, 5432/Postgres, 27017/MongoDB, 6379/Redis, 9200/Elasticsearch, 5900/VNC, and 3389/RDP — exposed databases and remote-desktop services are among the most heavily scanned and abused endpoints in the world.
Best Practices
Treat the Port Checker as part of your release checklist: after deploying a new service, confirm it's reachable from outside, then confirm that everything you didn't intend to expose is still closed. Use non-standard ports for personal services where allowed (it doesn't stop a targeted attacker but reduces noise from automated scanners by a huge margin). Always pair an open port with an authenticated, rate-limited service — never expose a database, queue, or admin panel directly. And document every open port in your network with the service name and an owner; orphaned open ports are how legacy systems get popped.
Troubleshooting Guide
Port shows closed but you're sure the service is running? Check the bind address — services bound to 127.0.0.1 only accept local connections. Port shows filtered? Walk the firewall path: cloud security group, host firewall (ufw, firewalld, Windows Defender), router NAT, ISP edge. Port shows open from this tool but not from the user's network? An intermediate ISP or corporate firewall is blocking; have them test from a phone hotspot to confirm. Port intermittently open? Often a DHCP-renewed router IP that broke port-forwarding. Port open but the service still seems unreachable? You may have a TLS or HTTP-level issue masking as a connectivity problem — try `curl -v` against the URL to see the actual error.
Frequently Asked Questions
What is a port checker?+
A port checker tests whether a specific port on a server or computer is open (accepting connections) or closed (blocking connections). It is used by network administrators and developers to troubleshoot firewall and connectivity issues.
Why is my port showing as closed?+
A port may appear closed because of a firewall rule blocking it, the service using that port is not running, or the router has not forwarded the port correctly.
Is it legal to check open ports?+
Checking ports on systems you own, run, or have written permission to test is fully legal. Scanning third-party systems without permission can violate the Computer Fraud and Abuse Act (US) and similar laws elsewhere.
What's the difference between closed and filtered?+
Closed means the host actively replied 'no service here'. Filtered means the host never replied at all — usually a firewall silently dropped the packet. Filtered ports are harder to debug because there's no acknowledgement either way.
Can this tool scan UDP ports?+
This tool checks TCP ports only, which covers HTTP, HTTPS, SSH, FTP, databases, and the vast majority of online services. UDP scanning is far less reliable because UDP services usually don't acknowledge probe packets.
Does an open port automatically mean my server is hacked?+
No. Open simply means a service is accepting connections. Whether that's safe depends on the service, its authentication, and how it's patched.
Why is port 25 blocked by my ISP?+
Most residential ISPs block outbound port 25 (SMTP) to prevent spam from infected home machines. Use a smart host or a transactional email provider instead.
Can I check UDP ports with this tool?+
Not reliably. UDP is connectionless, so most services don't reply to a probe — making 'no response' indistinguishable from 'no service'. This tool focuses on TCP for that reason.
What's the difference between a closed port and a filtered port?+
Closed means the host actively replied with TCP RST — the host is alive but nothing is listening. Filtered means no reply at all, which usually means a firewall dropped the probe.
Are well-known ports safer than high-numbered ports?+
No. Security depends on the service behind the port and its configuration, not on the number. Attackers scan the entire 1–65535 range.
How do I open a port on my router?+
Log into the router admin UI (often 192.168.1.1), find Port Forwarding, and forward the external TCP port to the internal IP and port of the host running the service.
Why is the check showing my own IP as filtered?+
Most consumer routers don't forward unsolicited inbound traffic to any device. Without an explicit port-forward rule, every inbound port on your public IP is filtered by default — which is the safe behavior.
How to check if a port is open
- 1
Identify the target
Enter the public hostname or IP of the server you want to test — never a private address like 192.168.x.x, which is unreachable from the internet.
- 2
Enter the port number
Type the TCP port from 1 to 65535. Common examples: 22 for SSH, 80 for HTTP, 443 for HTTPS, 25565 for Minecraft.
- 3
Run the check
The tool opens a TCP connection from our server to that endpoint and reports open, closed, or filtered.
- 4
Interpret the result
Open = a service is accepting connections. Closed = the host replied but nothing is listening. Filtered = a firewall silently dropped the probe.
- 5
Iterate on the diagnosis
If the port is closed or filtered, walk the four-step diagnosis: firewall, service running, router forwarding, ISP block.
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