4.1. Introduction to Linux Routing

4.1. Introduction to Linux Routing
The design of IP routing allows for very simple route definitions for small networks, while not hindering the flexibility of routing in complex environments. A key concept in IP routing is the ability to define what addresses are locally reachable as opposed to not directly known destinations. Every IP capable host knows about at least three classes of destination: itself, locally connected computers and everywhere else.
Most fully-featured IP-aware networked operating systems (all unix-like operating systems with IP stacks, modern Macintoshes, and modern Windows) include support for the loopback device and IP. This is an IP and range configured on the host machine itself which allows the machine to talk to itself. Linux systems can communicate over IP on any locally configured IP address, whether on the loopback device or not. This is the first class of destinations: locally hosted addresses.
The second class of IP addresses are addresses in the locally connected network segment. Each machine with a connection to an IP network can reach a subset of the entire IP address space on its directly connected network interface.
All other hosts or destination IPs fall into a third range. Any IP which is not on the machine itself or locally reachable (i.e. connected to the same media segment) is only reachable through an IP routing device. This routing device must have an IP address in a locally reachable IP address range.
All IP networking is a permutation of these three fundamental concepts of reachability. This list summarizes the three possible classifications for reachability of destination IP addresses from any single source machine.
The IP address is reachable on the machine itself. Under linux this is considered scope host and is used for IPs bound to any network device including loopback devices, and the network range for the loopback device. Addresses of this nature are called local IPs or locally hosted IPs.
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