Network fundamentals

Two or more devices connected so they can exchange data. That enables sharing hardware and files, communication between people, and reaching central services from anywhere.

A LAN covers one site on cabling the organisation owns, making it fast and cheap. A WAN spans several distant sites over links it does not own — rented lines or the internet — so it is slower and costs more per unit of data.

A personal area network: a few metres around one person, joining a phone to its earbuds or a smartwatch. Usually Bluetooth, chosen for very short range and very low power.

A virtual private network creates an encrypted tunnel across a public network, so a distant device behaves as though it were on the organisation's own LAN and can reach internal servers not exposed to the internet.

Benefit: one central copy of data that everyone works from and that stays current, plus shared hardware. Drawback: a single failure or break-in can affect everyone connected rather than one machine.

The internet is the global network of networks — the connections themselves. The web is one service running on it, made of pages fetched with HTTP. Email, calls and games are other services on the same internet.

Processing data where it is produced rather than sending it away. Right when an answer is needed in milliseconds, when raw data would flood the network, or when the device must keep working if its link fails — smart traffic lights, for example.

Many machines cooperating on one job with no single machine in charge. There is no single point of failure and it scales by adding machines, but keeping every machine agreed on the same answer is genuinely hard. A cryptocurrency blockchain is one.

Benefit: resources available on demand with no hardware to buy or maintain. Limitation: the service depends on both the provider's uptime and your own connection, and the costs recur indefinitely.

Bandwidth is shared with every device nearby, so a busy area slows down, and coverage has gaps where there is no signal from a mast.

A switch moves traffic within one network, delivering each frame to the device it is addressed to; it cannot read IP addresses. A router moves traffic between networks, reading the destination IP address to choose the next hop.

Joins two networks that use different protocols, translating between them so traffic can cross. A plain router only forwards; a gateway also converts.

A modem converts between the network's digital signal and what the physical line carries. A NIC connects one machine to the network. A wireless access point lets devices join the LAN over radio instead of cable.

Network access: switches, network interface cards, wireless access points and modems, handling frames and bits inside one network. Internet: routers and gateways, reading IP addresses to choose routes.

Sits where the network meets the outside world and inspects traffic against its rules, blocking anything forbidden — on outgoing traffic as well as incoming.

An agreed set of rules for a conversation between machines: what gets said, in what order, and what a reply means. Machines from different makers can talk only because both follow the same rules.

TCP numbers packets, checks they all arrived, asks again for missing ones and reorders them — so data arrives complete. UDP sends without checking and never resends, so it is faster but packets may be late, out of order or lost.

When timeliness matters more than completeness — live video, voice calls, online games. A late packet is already useless, so waiting for a resend delays everything after it for no benefit.

Two guarantees: encryption, so nobody between the two ends can read the conversation, and identity, because a digital certificate proves the server really is who it claims to be.

Giving a device an IP address when it joins a network. The device asks, and a DHCP server assigns one from its pool for a limited time, instead of every device being configured by hand.

**Application** (what it means) · **Transport** (did it all arrive) · **Internet** (which host) · **Link** (across this medium).

Each layer **wraps** what it was given in its own header on the way down, and **removes its own** on the way up.

The **MAC address** is rewritten at every hop, because it names only the next device on this link. The **IP address is unchanged** end to end.

**UDP.** TCP would retransmit a lost frame and hold everything behind it — but by then the frame is already in the past, and the pause freezes the call.

Each layer depends only on the **interface** of the one below, so swapping Wi-Fi for Ethernet changes nothing above the link layer.