The scenario
Sender (10.0.1.10) talks to Receiver (10.0.3.10, a web server on TCP port 80) through Router-1 and Router-2. Router-1 has a static route to Receiver's network, 10.0.3.0/24 via 10.0.2.2, so requests get all the way there. Router-2 has no route to Sender's network, 10.0.1.0/24. Receiver answers every request, and Router-2 drops every answer. From Receiver's side traffic is arriving; from Sender's side the server never responds. OSPF is switched off on both routers, so nothing fills in the missing route.
What you will work out
Understand that communication needs a forward path and a return path, and learn to find the router where the return path ends.
Why this matters
A missing return route produces some of the most confusing incidents: the server's logs or a capture show requests arriving, the client reports timeouts, and the forward path checks out perfectly. Experienced engineers test from both ends and check the return route on every router in the path. Routing protocols avoid most of these by advertising networks automatically, while static routes have to be added on every router, in both directions, by hand.
Running it
Packets Enter But Never Leave 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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