Port Checker

When a service will not connect, the question is almost always where the traffic is being stopped. A port reference helps you reason about that: knowing that 443 is HTTPS, 22 is SSH, 3306 is MySQL and 25 is SMTP tells you what a blocked or unexpected port means before you start changing configuration.

Ports are how a single machine runs many services on one address. The address gets the packet to the host; the port number gets it to the right process. Firewalls, routers and cloud security groups all filter on that number, which is why a service can be running perfectly and still be unreachable.

Use the reference when planning firewall rules, when reviewing which ports a host exposes, or when documenting a system. Follow the principle of least exposure: open only the ports a service genuinely needs, keep administrative interfaces such as SSH and database ports off the public internet entirely, and put them behind a VPN or bastion host instead.

How to use the Port Checker

1. Identify the port number your service uses.
2. Look up what that port is conventionally used for.
3. Confirm the service is actually listening on the host.
4. Check each firewall between client and server, including cloud security groups.
5. Open only the ports required, and restrict the source addresses where possible.

Examples

• An administrator plans firewall rules for a new web server exposing only ports 80 and 443.
• A developer diagnoses why a database connection fails and finds port 5432 blocked at the cloud security group.
• A security reviewer audits which ports a host exposes and closes an administrative interface left open.

Frequently asked questions

What are the well-known ports?

Ports 0 to 1023 are reserved for standard services such as HTTP on 80, HTTPS on 443, SSH on 22 and SMTP on 25.

What is the difference between TCP and UDP ports?

They are separate number spaces. TCP is connection oriented and reliable; UDP is connectionless and used for DNS, streaming and games.

Should I change SSH to a non-standard port?

It reduces automated scanning noise but is not real security. Key-based authentication and restricted source addresses matter far more.

Why is my port closed even though the service is running?

Something between client and server is filtering it: a host firewall, a router, a cloud security group, or a service bound only to localhost.

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