9.6 The Default Gateway
The entry for default in the routing table has special significance because it matches any address on the internet. In CIDR notation, it’s 0.0.0.0/0 for IPv4. This is the default route, and the address configured as the intermediary in the default route is the default gateway. When no other rules match, the default route always does, and the default gateway is where you send messages when there is no other choice. You can configure a host without a default gateway, but it won’t be able to reach hosts outside the destinations in the routing table.
On most networks with a netmask of /24 (255.255.255.0), the router is usually at address 1 of the subnet (for example, 10.23.2.1 in 10.23.2.0/24). This is simply a convention, and there can be exceptions.
There’s one tricky detail in routing. Say the host wants to send something to 10.23.2.132, which matches both rules in a routing table, the default route and 10.23.2.0/24. How does the kernel know to use the second one? The order in the routing table doesn’t matter; the kernel chooses the longest destination prefix that matches. This is where CIDR notation comes in particularly handy: 10.23.2.0/24 matches, and its prefix is 24 bits long; 0.0.0.0/0 also matches, but its prefix is 0 bits long (that is, it has no prefix), so the rule for 10.23.2.0/24 takes priority.