1.3.2. The Routing Decision Framework

When a device needs to send data, it makes a fundamental binary decision that determines everything else:

How devices determine "local": The subnet mask defines what's local. With IP 10.0.0.5/24 and mask 255.255.255.0, anything matching 10.0.0.x is local. 10.0.0.100 is local (same first 24 bits). 10.0.1.100 is remote (different third octet).

This framework explains:

• Why default gateway must be configured: Without it, devices can't reach anything "remote"
• Why subnet masks matter: They define the boundary between "local" and "remote"
• Why misconfigured masks break connectivity: A wrong mask makes devices think remote hosts are local (or vice versa), sending traffic to the wrong place