# Network – Basics

Core networking concepts: IP addressing, subnets, ports, routing, and protocols.

Related pages:

- [Network / DNS](./DNS.md)
- [Network / Reverse Proxy](./ReverseProxy.md)
- [Network / SSH](./SSH.md)
- [Network / VPN Basics](./VPN/Basics.md)
- [Network / Firewall](./Firewall.md)

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## 1. IP Addresses

Every device on a network is identified by an **IP address**. The most common version is IPv4, written as four numbers separated by dots: `192.168.1.18`. Each number is between 0 and 255, giving 4 bytes (32 bits) per address.

### 1.1 Private vs Public Addresses

Some ranges are reserved for private use — they are only valid inside a local network and are never routed on the public internet:

| Range             | Example         | Common use                   |
|-------------------|-----------------|------------------------------|
| `10.0.0.0/8`      | `10.10.10.1`    | VPNs, corporate networks     |
| `172.16.0.0/12`   | `172.16.0.1`    | Docker default bridge        |
| `192.168.0.0/16`  | `192.168.1.18`  | Home/office LANs             |

Everything else is a **public** address, routable on the internet. Your internet box has one public IP assigned by your ISP.

### 1.2 Subnets and CIDR Notation

A subnet groups a range of IP addresses together. The size is expressed with **CIDR notation**: a `/` followed by the number of fixed bits.

- `/24` → first 24 bits are fixed → 256 addresses (`192.168.1.0` to `192.168.1.255`).
- `/32` → all 32 bits are fixed → exactly one address.
- `/16` → first 16 bits are fixed → 65 536 addresses.

The most common home network is a `/24`, for example `192.168.1.0/24`.

A **subnet mask** expresses the same thing differently: `/24` ↔ `255.255.255.0`.

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## 2. Ports

An IP address identifies a machine. A **port** identifies a specific service or application running on that machine. Ports are numbers from 0 to 65 535.

When two machines communicate they use an IP + port pair: `192.168.1.18:80`.

### 2.1 Why Ports Exist

A server typically runs several services at once — a web server, an SSH daemon, a database, etc. Ports allow the OS to route each incoming packet to the correct service.

- A packet arrives at `192.168.1.18:443` → the OS delivers it to the HTTPS server.
- A packet arrives at `192.168.1.18:22` → the OS delivers it to the SSH daemon.

When you "open a port", you are telling a firewall or router to allow traffic destined for that port number to pass through.

### 2.2 TCP vs UDP

| Protocol | Characteristics                                              | Common uses                               |
|----------|--------------------------------------------------------------|-------------------------------------------|
| **TCP**  | Connection-oriented, reliable, ordered delivery             | HTTP/HTTPS, SSH, databases, email         |
| **UDP**  | Connectionless, no delivery guarantee, lower overhead       | DNS, WireGuard, video streaming, QUIC     |

TCP establishes a connection before transferring data and retransmits lost packets. UDP fires packets without verifying receipt — faster but not guaranteed.

### 2.3 Well-Known Ports

Ports 0–1023 are "well-known" — standardised assignments used by common services:

| Port  | Protocol | Service                  |
|-------|----------|--------------------------|
| 22    | TCP      | SSH                      |
| 53    | TCP/UDP  | DNS                      |
| 80    | TCP      | HTTP                     |
| 443   | TCP      | HTTPS                    |
| 51820 | UDP      | WireGuard (conventional) |

Ports 1024–49 151 are "registered". Ports 49 152–65 535 are "dynamic" (used temporarily for outbound connections).

For filtering and controlling port access, see [Network / Firewall](./Firewall.md).

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## 3. Routing

Routing is the process of forwarding packets from one network to another. At home this is mostly transparent:

1. Your device sends a packet.
2. If the destination is on the same subnet, it is delivered directly.
3. Otherwise it is sent to the **default gateway** (your router), which forwards it towards the internet.

The default gateway is normally the IP of your router on the local network (for example `192.168.1.1`).

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## 4. HTTP and HTTPS

**HTTP** (HyperText Transfer Protocol) is the protocol used by web browsers and APIs. It is a request-response protocol over TCP:

- A client sends a request: `GET /page HTTP/1.1`
- A server responds with a status code and a body: `200 OK`

**HTTPS** is HTTP over a **TLS** (Transport Layer Security) encrypted connection. TLS:

- Encrypts traffic so it cannot be read in transit.
- Authenticates the server via a certificate.

Modern browsers require HTTPS. Self-signed certificates work for internal networks but require the client to explicitly trust the issuing CA.

For TLS and certificates in depth, see:

- [Security / Certificates](../Security/Certificates.md)

For HTTP routing via a reverse proxy, see:

- [Network / Reverse Proxy](./ReverseProxy.md)