IP routing

How do packets get from one place to another (when they do)?

Hosts have tables listing available network interfaces and the IP networks reachable from them, containing:

• A route destination and netmask (to specify a particular range of IP addresses reachable from this route)
• A gateway (the IP address to forward traffic to, if relevant)
• An interface (the hardware interface for that network)
• Flags (used internally to specify route types and other information)

A netmask is literally just a bitmask used with logical operations to manipulate address bits

• It's usually written either like a numeric IP address (255.255.254.0) or in the more modern "CIDR" notation as /23 (the number of one bits on the left); both stand for a mask of 11111111111111111111111000000000
• For routing decisions, if dest_IP & netmask == route_dest then the destination IP is considered to match that route
• For sending broadcast messages, the broadcast address for a subnet is dest_IP | ~(netmask)

A router is just a host that has multiple physical network interfaces and knows how to move packets between multiple networks

Both routers and normal hosts have routing tables, but a router's table is usually more complicated

Next ->