What is a routing table and what information does it contain?

The routing table maps destination networks to "send it out interface X (directly, or via gateway Y)"; the most specific match wins, and 0.0.0.0/0 is the catch-all default.

* The kernel picks the route whose prefix most specifically contains the destination (longest-prefix match); 0.0.0.0/0 is only the catch-all default. *

For every outgoing packet the kernel consults this table to answer "where next?". It looks for the entry whose network most specifically contains the destination (longest-prefix match). If the destination is on a directly-connected network, it's delivered straight out that interface; otherwise it's handed to a gateway (a router) to forward onward.

Three concepts to hold:

• Direct route — destination is on a locally-connected subnet, no gateway needed
• Gateway route — destination is elsewhere, so forward to a router's IP
• Default route (0.0.0.0/0) — matches anything not matched above; this is "send everything else to the internet via my router"

View it:

ip route     # modern (iproute2)
route -n     # legacy

Tip: the default route's gateway is your router's LAN IP — lose that entry and you can reach your local subnet but nothing on the wider internet.

Go deeper:

• ip-route(8) man page (man7.org) — the iproute2 reference: default (0/0), via gateway, and longest-prefix routing.