The scenario
PC-Left (10.0.1.10/24) and PC-Right (10.0.2.20/24) are on different subnets joined by Router (10.0.1.1 on PC-Left's side, 10.0.2.1 on PC-Right's side). The Router is configured correctly, and PC-Right has its default gateway, 10.0.2.1. PC-Left has none. Your job is to trace, read where each conversation stops, and fix it.
What you will work out
Discover why a host without a default gateway cannot talk to other subnets, in either direction.
Why this matters
A missing or wrong default gateway is one of the most common reasons a device can reach its own network and nothing else. It is also easy to misdiagnose from the other end: PC-Right's ping simply times out, which says nothing about which device is at fault. The pattern here works everywhere: trace, see which device stopped the conversation, and read that device's routes.
Running it
Missing Gateway: Why Hosts Need Routes opens on a canvas, already wired and addressed. Trace a packet through it hop by hop, or run the same topology live on real Linux network namespaces. How the two modes differ.
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