Part 1 — The scenario

The story

It's 9:14 pm. You're on the sofa with your laptop, connected to home Wi-Fi. Your cart on shop.example.com holds a pair of running shoes. You click Place order.

The browser's JavaScript runs:

await fetch("https://shop.example.com/api/v1/orders", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    "Idempotency-Key": "7f3c9a2e-1b4d-4c8e-9f00-2a6b8d1e5c43",
  },
  body: JSON.stringify({ cartId: "c_8812", paymentMethodId: "pm_77" }),
});

From your point of view that's one line of code. Underneath, it involves about a dozen devices, three or four organisations, two continents' worth of routing tables, and every layer of the OSI model, twice: once on the way out and once on the way back.

The cast of characters

All addresses use reserved documentation ranges, so they're safe to copy.

#	Who	What it is	Key addresses
1	Your laptop	The client: browser, OS kernel, Wi-Fi chip	MAC `3c:22:fb:aa:bb:01`, IP `192.168.1.23`
2	Home router	Wi-Fi access point + switch + router + NAT + DHCP server + DNS forwarder in one box	LAN MAC `9c:53:22:00:00:01`, LAN IP `192.168.1.1`, WAN IP `203.0.113.7`
3	ONT (Optical Network Terminal)	The small box that turns the fibre's light into Ethernet	(no IP you care about)
4	ISP access network	OLT, aggregation switches, BNG (Broadband Network Gateway)	Your gateway as seen from the ISP
5	ISP core and the Internet	Routers that speak BGP to other networks	Many hops
6	Cloud edge	The cloud provider's border routers
7	Load balancer (ALB)	An L7 proxy that terminates TLS and routes by path	Public `198.51.100.20`, private `10.0.1.15`
8	Kubernetes node	A VM running your pods	`10.0.2.40`
9	`orders-service` pod	Your code	`10.0.2.77:8080`
10	Postgres	The database	`10.0.3.10:5432`
11	DNS resolvers	Recursive resolver, root, `.com` TLD, `example.com` authoritative servers	Various

The map

 YOUR HOME                         ISP                          INTERNET              CLOUD REGION (VPC 10.0.0.0/16)
┌─────────────────────────┐   ┌────────────────────┐   ┌────────────────────┐   ┌──────────────────────────────────────────┐
│ Laptop                  │   │                    │   │                    │   │  ┌─────────┐   ┌──────────┐   ┌────────┐ │
│ 192.168.1.23            │   │ OLT ─ Aggregation  │   │  Transit / peering │   │  │   ALB   │──▶│ K8s node │──▶│  Pod   │ │
│     │  Wi-Fi (radio)    │   │        │           │   │   routers (BGP)    │   │  │ L7 proxy│   │10.0.2.40 │   │10.0.2.77│ │
│     ▼                   │   │       BNG ─ Core ──┼──▶│                    │──▶│  │10.0.1.15│   └──────────┘   └───┬────┘ │
│ Home router 192.168.1.1 │   │                    │   │                    │   │  └─────────┘                      │      │
│ NAT → 203.0.113.7       │   └────────────────────┘   └────────────────────┘   │   ▲ 198.51.100.20         ┌───────▼────┐ │
│     │  Ethernet (copper)│          ▲ fibre (light)                            │   │                       │ Postgres   │ │
│     ▼                   │          │                                          │   │                       │ 10.0.3.10  │ │
│ ONT ════════════════════┼──────────┘                                          │                           └────────────┘ │
└─────────────────────────┘                                                     └──────────────────────────────────────────┘

Keep this map in mind. Every layer chapter comes back to it.