Loopback — 127.0.0.1 and the kernel's own network stack

The first check in every ladder: can this machine talk to itself? If not, no cable in the world will help.

What it is. 127.0.0.1 (and ::1) never leaves the machine. Packets to it go into the kernel's network stack and straight back out, without touching a network card. It's the same code path every local service uses — a database on localhost:5432, a web UI on 127.0.0.1:8080.

What the check proves. If loopback answers, the TCP/IP stack itself is fine. Anything failing further up the ladder is a cable, a router, an ISP or a name — not this computer's networking code.

Good looks like: a reply in well under a millisecond.

When it's red

• Linux: the lo interface is down. ip link show lo; bring it up with sudo ip link set lo up.
• A broken hosts file: localhost must map to 127.0.0.1. Check /etc/hosts or C:\Windows\System32\drivers\etc\hosts — n host . audits it.
• A firewall rule on loopback: rare, but a mis-typed iptables/ufw rule or a Windows firewall profile can block lo. Nothing legitimate ever needs to.
• Windows Winsock corruption (after some VPN or antivirus uninstalls): netsh winsock reset and netsh int ip reset, then reboot.