Default gateway — the router, and the route to everything else

Every packet that isn't for your own subnet goes to the gateway. No route, or no reply, and the internet is unreachable no matter what else works.

What it is. The gateway is your router's LAN address (usually 192.168.1.1 or 192.168.0.1). The *default route* says: "anything not on my own subnet, send it here". Your machine learned both from DHCP when it joined the network.

Good looks like: a reply in about 1 ms on Ethernet, 2–10 ms on Wi-Fi.

When it's red

• "No default route found" — DHCP failed. On Windows the address will be 169.254.x.x (APIPA), on Linux the interface may have no address at all. Renew: ipconfig /renew or sudo dhclient -r && sudo dhclient. If it fails again, the router's DHCP is off or the link is dead.
• "Router unreachable" — the route exists but the router doesn't answer. On Ethernet: cable, switch port, or the router itself. On Wi-Fi: you're associated with the access point but nothing beyond it works — try a different band or reboot the AP.
• High latency to the gateway (tens of ms) on a LAN means Wi-Fi contention or a saturated router. A wired machine at 20 ms to its own router is a problem with the router, not the internet.

See for yourself

• Linux/macOS: ip route (Linux) or netstat -rn (macOS) — look for default via.
• Windows: route print — the 0.0.0.0 row.
• n ping . runs this ladder; n check <gateway-ip> treats the router as a host.