The scenario
Client (10.0.1.10/24) and Server (10.0.2.20/24, a web server on TCP port 80) are on different subnets, so neither can deliver to the other directly. Router has one interface in each network: ge0 is 10.0.1.1 on the Client's side and ge1 is 10.0.2.1 on the Server's side. Each host has a default route pointing at the Router's address on its own subnet. That default route, the default gateway, is the host's rule for everything that is not local: hand it to the router.
What you will work out
See why hosts need a default gateway and how a router moves packets between two LANs.
Why this matters
Every time a laptop opens a website, it hands the packet to its default gateway, the home or office router, which passes it on toward the destination. When a device can reach its own subnet but nothing beyond, the default gateway is the first thing to check: does the host have one, and is it the router's address on the host's own subnet?
Running it
Router Between Two Networks 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.
Open Router Between Two Networks
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