Lesson 01 · Foundations

What a network is

Start from zero: a laptop, a printer and a phone in one home. Join them with links and they become a network. Watch a print job get cut into packets that hop through a switch, then follow a phone's packet out through the router onto the internet, a network of networks.

8 min9 steps5 questionsBeginner

Fig 1 · Interactive

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5 quick questions

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01What turns three separate devices into a network?

A network is nodes plus links. Until a link joins them, devices cannot share anything, however close they sit.

02Why does the laptop cut the print job into packets?

Packets have a maximum size (1,500 bytes on a wired home LAN). Small pieces let many devices share one link fairly; the receiver puts them back in order.

03What does a switch do on a home LAN?

A switch joins devices inside one LAN. Leaving the LAN is the router's job.

04Which of these is a LAN?

A local area network covers a small area such as a home or office. A provider's network spanning towns and countries is a WAN.

05Why is the internet called a network of networks?

No single network is the internet. Routers join each network to its neighbours, and packets are handed from network to network.

Key terms

Glossary

Node: Any device on a network that can send or receive data: a laptop, a phone, a printer, a server.
Link: A path between two nodes: a cable, or radio waves such as Wi-Fi.
Network: Nodes joined by links so they can share data.
Packet: A small piece of a message with a label saying who sent it and who it is for.
Switch: A box that joins the wired devices of a LAN and passes packets between them.
Router: A box that joins one network to another and chooses which way to send each packet.
LAN: Local area network: devices close together, such as in one home or office.
WAN: Wide area network: a network spanning towns, countries or continents, such as an internet provider's.

Fine print

Notes and sources

1,500 bytes is the standard Ethernet maximum (RFC 894); 40 bytes of each packet go to the smallest IPv4 and TCP labels (RFC 791, RFC 9293), leaving 1,460 for the job. Real print jobs are usually much larger; this one is tiny so every packet fits on screen.
A switch sends a packet only out of the printer's port once it has learned where the printer is; before that it copies it to every port. The "How a switch learns" lesson shows this.
192.168.1.0/24 is a private home range (RFC 1918); 203.0.113.5 and 198.51.100.20 are RFC 5737 documentation addresses. Addresses get their own lessons later.
Counting every home and office LAN, the internet joins hundreds of millions of networks; the large independently run ones (autonomous systems) number in the tens of thousands.

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