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Network Debugging Notes

Last updated: 27 September 2026

“The service is down” can mean several things: DNS points to the wrong address, the port cannot be reached, the connection works but TLS fails, or the application replies with an error. Find the first step that breaks.

Follow the path

Resolve the hostname, check the address you received, then test whether the expected port accepts a connection. If it does, inspect the protocol response. A reachable TCP port does not prove the HTTP application is healthy; a working local request does not prove the public path through a firewall or proxy works.

Compare vantage points

Test from the machine running the service and from the client that cannot reach it. Check what address the process listens on: 127.0.0.1 only accepts local IPv4 connections, while 0.0.0.0 listens on available IPv4 interfaces. Be careful with public lookups; DNS and IP tools may send your query to an external service.

  • DNS Lookup — Resolve A, AAAA, MX, TXT, NS, SOA, CAA and SRV records over DNS-over-HTTPS.
  • IP & Geolocation Lookup — Show your public IP (or any address) with its country, city, coordinates and ISP.
  • IPv4 Address Converter — Convert between dotted, decimal, hex and binary IPv4 forms.
  • IPv4 Range Expander — Expand an IP range into the CIDR blocks that cover it.
  • IPv4 Subnet Calculator — Work out the network, broadcast, mask and host range for a CIDR block.
  • IPv6 Expander — Expand, compress and classify an IPv6 address, with prefix and embedded IPv4 support.
  • IPv6 ULA Generator — Generate a random IPv6 unique local address prefix.
  • MAC Address Generator — Generate random MAC addresses, with optional prefix.
  • MAC Address Lookup — Look up the vendor behind a MAC address OUI.
  • Network Info — Show what this browser exposes about its connection, device, screen and locale.
  • Port Reference — Search the common TCP and UDP ports with their service names and a short description.
  • Random Port Generator — Pick one or more unused-looking port numbers.
  • SSH Tunnel Builder — Assemble an ssh -L, -R or -D tunnel with the right ports, identity file and keepalive options, then copy the command.
  • WHOIS Lookup — Show registrar, registration and expiry dates, status and nameservers for a domain.
  • Wi-Fi QR Code — Build a QR code that joins a Wi-Fi network.