Build it: ping, and the parcel that dies on purpose (ICMP and traceroute)

A parcel passes depot after depot, and each depot only knows the next one. This lesson builds the two tools that make that journey visible: one that asks “are you there, and how far?”, and one that makes every depot on the way give its name.

Before you start. You need How a parcel finds any house on earth, which did traceroute by hand: depots, hops, and the hop limit that stops a parcel circling forever. Inside one postal company (distance-vector, link-state, and ICMP) met ICMP, the notes routers send back, and showed ping and traceroute output without building either. Here you write both, in Python, with no root access.