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Networks

A computer network links devices so they can exchange data, over copper cable, radio (wireless LANs) or optical fibre. Data travels as packets, each carrying addresses, and is handled by a stack of protocol layers from the physical link up to applications. Routers join separate networks together by forwarding each packet on its own, while the end devices keep track of reliability.

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Packet switching and statelessness

Packet switching (theory first published by Kleinrock in 1961, developed in parallel at RAND and the UK's NPL) shares link capacity among many flows. In the Internet architecture (RFC 1122), routers are stateless: each IP datagram is forwarded independently, so redundant paths can be used when a router or link fails.

  • Link-layer frames carry IP datagrams one hop at a time; link addresses (EUI-48) are only meaningful on the local link.
  • IP addresses are end-to-end identifiers used for routing between networks.
  • End-to-end reliability, ordering and flow control are implemented by hosts (e.g. TCP), not routers.
loss (dB)=−10log⁡10 ⁣(PoutPin)\text{loss (dB)} = -10 \log_{10}\!\left(\dfrac{P_{\text{out}}}{P_{\text{in}}}\right)

Attenuation in decibels; 95% transmission over 1 km is about 0.22 dB/km (our calculation).

Common misconception: Layering is a design discipline, not a physical stack of boxes; real systems sometimes cross layers for performance, at the cost of flexibility.
Full explanation — the complete reference version every reading depth is based on

What a network is

A network is a set of devices — computers, phones, printers, sensors — connected so they can send data to one another. Links may be wired, as in Ethernet (standardised by the IEEE 802.3 Working Group), wireless, as in wireless local area networks (IEEE 802.11, sold under the Wi-Fi name), or optical fibre, which carries data as light.

Packets instead of a reserved line

Rather than reserving a continuous connection for each conversation, data networks split messages into packets that share the links with everyone else's packets. Leonard Kleinrock published the first paper on packet-switching theory in July 1961. Similar work went on at the same time, independently, at RAND in the USA and at the National Physical Laboratory in the UK, where the word 'packet' came from. These ideas became the basis of the ARPANET and then the Internet.

Layers

  1. Application layer: the programs' own protocols, such as web, email or name lookup.
  2. Transport layer: end-to-end delivery between programs, for example TCP (reliable) or UDP (connectionless).
  3. Internet layer: the Internet Protocol (IP), which moves datagrams from source host to destination host across networks.
  4. Link layer: the protocol for the directly connected network, such as Ethernet or a wireless LAN.

Each layer relies on the one below and offers a service to the one above, so a web browser does not need to know whether its data travels over radio, copper or glass.

Addresses and routers

  • Network hardware interfaces usually carry a 48-bit identifier (EUI-48, often called a MAC address) used on the local link.
  • IP addresses identify hosts across interconnected networks.
  • Routers (called 'gateways' in the original specifications) join networks and forward each IP datagram independently of the others.
  • Reliability and flow control are handled end to end by the hosts, not by the routers in between.

Worked example: light in a fibre

Before Kao's work in the 1960s, only 1% of the light entering a glass fibre survived 20 metres. Charles Kao and George Hockham aimed for at least 1% to remain after 1 km (Kao presented their conclusions in 1966). By 2009 about 95% remained after 1 km. If each kilometre keeps 95%, then after 3 km 0.95 × 0.95 × 0.95 ≈ 0.857, so about 86% remains (our calculation) — which is why long fibre links still need optical amplifiers along the way.

P(L)=P0×0.95L/kmP(L) = P_{0} \times 0.95^{L/\text{km}}

Power remaining after L kilometres if each kilometre keeps 95% of the light (illustrative model).

How we know

The layered architecture and the stateless-router principle are written down in the Internet's own standards (RFC 1122, 1989). The history comes from 'A Brief History of the Internet', written by people who built it, including Kleinrock himself. The fibre figures come from the Nobel Prize committee's account of Kao's work.

Assumptions and limits

The four-layer picture is a design discipline: real devices sometimes cross layers, and middleboxes such as firewalls and address translators do keep per-connection state. The 95%-per-kilometre model ignores splices, connectors and dispersion, which also limit real links.

Common misconception: Misconception: 'wireless means the data never travels through cables'. A phone's Wi-Fi link is only the first hop; beyond the wireless access point, packets usually continue over cables and optical fibre.
Info: Connections: the Internet is a network of networks built on these ideas, every networked device is a Computer with input and output hardware for its link, and optical fibre puts the physics of Light to work.

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Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
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