Mental Model — Hosts, Routers, and Layers

Before writing a single line of code, it helps to picture where your stack sits in the grand scheme of the Internet.

Every packet travels through a chain of machines. Each machine looks at only the information it needs for its role:

[ Your client ] → [ Router / Gateway ] → [ Remote host ]
 ↑ ↑ ↑
 TCP/UDP, App IP + Routing TCP/UDP, App
 (L4–L7) (L3) (L4–L7)

Roles in the Network

A Host

Like your laptop or a web server, runs all layers up to the application. It creates TCP connections, talks HTTP, DNS, SSH, and so on.

A Router

Operates mostly at Layer 3 (Network). It doesn't "understand" HTTP or TCP streams; it just forwards IP packets between networks, maybe adjusting headers or performing NAT.

Your Stack Will Be Both

In this course, your code will act as:

1. A router/gateway when it forwards traffic from one interface to another
2. A host when it terminates packets destined for itself (e.g., replying to ping or serving HTTP)

The Packet Journey

So when you run:

curl http://10.10.0.1:8080/

The request leaves the client's kernel stack as real Ethernet frames, crosses a virtual network bridge, and lands in your container.

Your stack then walks the packet up the layers:

Ethernet → IP → TCP → HTTP

And the response goes back down the same path in reverse.

Success: That's the journey you'll trace and implement, step by step.