ip netns

Also written: ip netns exec, namespace, network namespace, ip -n

Linux network namespaces: separate, pretend computers on one machine. ip netns exec laptop CMD runs a command inside the laptop namespace, and ip -n laptop is a short form for ip commands.

Where the book teaches it

Start here
Chapter 1: Ports and the transport layer
Chapter 3: How DNS works
Chapter 10: Project: the full Kettle & Crane office