Chapter 30. Routing: Concepts
Figure 30-1 shows where the routing subsystem (the gray box) fits into the network stack. The figure does not include all the details (Netfilter, bridging, etc.) but shows the other major kernel subsystems that are traversed before and after routing.
Figure 30-1. Relationship between the routing subsystem and the other main network subsystems
To explain some of the features or the details of their implementation, I'll often show snapshots of user-space configurations. You are encouraged to use Chapter 36 as a reference if you need to learn more about the user-space tools I employ in the examples.
The discussion on routing will focus on IPv4 networks. However, I will point out the aspects of IPv6 that differ significantly.