How do you check if a port is open?

Direct answer

To check if a port is open from the internet, run a TCP connect test from an external host (or monitoring probe) to host:port. Locally, tools like Test-NetConnection (Windows) or nc/telnet help, but external validation matters for firewall and security group rules.

Interpretation

• Open: Connection completes (may still reject at TLS/app layer depending on protocol).
• Filtered: Timeout or silent drop—often firewall or ISP blocking.
• Closed: RST/refused—nothing listening or explicitly denied.

Pitfalls

Testing only from inside your VPC; confusing ICMP availability with TCP service availability; assuming port open equals application healthy.

Frequently asked questions

UDP?

Harder—many services are request/response specific; prefer app-level probes when possible.

Cloud security groups?

Remember separate rules for IPv4/IPv6 and different load balancer fronts.