39D The 39D Network ToolboxPublic network diagnostics and routing intelligence

CONNECTIVITY

Limited TCP Port Checker

Check whether selected common TCP ports accept a connection. Private targets, arbitrary ports and large scans are blocked.

39DExternal testRuns from the toolbox server

RUN TEST

Limited TCP Port Checker

Public targets only

Allowed ports: 21, 22, 25, 53, 80, 110, 143, 443, 465, 587, 993, 995, 3389, 5060, 5061, 8080, 8443. The public checker does not scan ranges or arbitrary ports.

HOW IT WORKS

About the Limited TCP Port Checker

The server attempts a short TCP connection only to allow-listed ports. It does not fingerprint services, send protocol payloads or scan port ranges.

How to use this result

  1. Run the test from this server.Use a public hostname, address, prefix or ASN in the format requested above.
  2. Compare with another observation point.Run a local command or use a second provider to identify location-specific behaviour.
  3. Correlate related evidence.Use the related tools below to compare DNS, routes, ports, TLS and application responses.
  4. Keep the time and context.Routing, DNS caches and reputation data change, so record when the result was collected.

Common interpretation issues

  • An open port does not prove the service is correctly configured.
  • Firewalls may silently drop probes, appearing as a timeout.
  • Source-IP allow-lists can make the result differ from other locations.

NEED HELP INTERPRETING THE RESULT?

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For ongoing support or a larger infrastructure project, speak to the 39D team.

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Frequently asked questions

Why can I not enter any port?

The public service deliberately restricts ports and request size to reduce abuse.

Does closed mean the host is offline?

No. It only means that connection did not succeed from this server.