A network is a set of machines wired so they can pass data to each other. One thing trips people up: when does traffic go out to the internet, and when does it stay on your own network? Send some traffic and watch where it goes.
Try it · send traffic and watch where it goesEvery device on a network gets an IP address: a number that says where it is. An address has two parts, a network part shared by everything on the same network, and a host part unique to each device. The prefix (the /24) marks where one part ends and the other begins.
People remember names; machines route by numbers. DNS is the lookup that turns a name into an IP address. Look one up and watch the lookup travel from server to server until it finds the address. The steps are how real resolution works; the addresses are examples.
Try it · look up a namenode.local never leaves your network. A known tracker is refused by your own resolver. Try all three.One machine runs many services, so each listens on a numbered door called a port. A port is only reachable when two conditions are met: a service is listening on it, and the firewall lets the connection through. Toggle each and watch what an outsider can reach.
Try it · toggle the two gates on each portThis is what default-deny means: the firewall gate starts closed on every port, so a service is never exposed just because it is running. You open a port only for a service you need, and only to the devices you trust.
This reference is not needed to build the node. It covers the layer underneath: how the pieces are modeled, what a packet carries, how addresses are carved up, and how networks are shaped.
Two models describe the same journey. The 7-layer OSI model is the teaching reference; the 4-layer TCP/IP model is what the internet actually runs. They map onto each other.
| OSI layer | TCP/IP | Does | Example |
|---|---|---|---|
| 7 Application | Application | The app's own data and rules | HTTP, DNS, SSH |
| 6 Presentation | Encoding and encryption | TLS | |
| 5 Session | Keeps a conversation open | sockets | |
| 4 Transport | Transport | Splits into segments, ports, reliability | TCP, UDP |
| 3 Network | Internet | Addresses and routes packets | IP, ICMP |
| 2 Data Link | Link | Frames between adjacent devices | Ethernet, MAC |
| 1 Physical | The wire or radio itself | cabling, wifi |
Your data does not travel alone. Each layer wraps it in a header, like nested envelopes. Click a segment to see what it carries.
A prefix splits an address block into a network and the hosts inside it. Enter an address and prefix to see the real numbers.
Usable hosts = 2^(32 − prefix) − 2 (the network and broadcast addresses are reserved). A /24 gives 254; a /30 point-to-point link gives 2.
This section covers how the machines are wired together. Your build uses a star.
Every device connects to one central point, your switch and router. It is simple and the standard layout for a home or small network.
Devices connect to many others, so traffic has more than one path. It is resilient, but requires more wiring and overhead.
All devices share one line. It is cheap and old; one break can take down the segment.
Each device links to two neighbors in a loop. Some backbone networks use it; traffic routes around any break.