Nodes and links
A laptop, a printer and a phone sit in one home. Each is a node: a device that can send or receive data. On their own they share nothing; the laptop has no way to reach the printer.
A link is any path between two nodes: a cable, or radio waves like Wi-Fi. Cables join the laptop and printer to a switch, a box that passes messages between the devices plugged into it. Nodes plus links make a network.
Packets
You press Print. The laptop does not send the job in one piece: it cuts it into small chunks called packets. Each packet carries a label saying who sent it and who it is for, here the printer.
The packets travel one after another down the cable to the switch. Because they are small and take turns, one big job cannot hog the network: other programs and devices get their turn in between.
The switch reads each label and sends the packet out of the one port the printer is on, not to every device. The printer puts the packets back in order and prints the page.
LAN, router, internet
Everything in this home is one LAN, a local area network: devices close together, joined by your own cables and Wi-Fi. The laptop reached the printer without anything ever leaving the house.
The phone wants a web page that lives on another network, far away. Its packet goes over Wi-Fi to the router, the box that joins your LAN to other networks and knows which way to send things.
The router passes the packet to your provider's network, a WAN or wide area network that spans towns and countries. Routers there hand it from network to network until it reaches the one holding the web page.
The internet is not one machine. It is millions of networks, from homes and offices to providers and data centres, joined to each other by routers. Your LAN is one small piece: the internet is a network of networks.
What is happening
;; step 1/9 · nodes ;; about: laptop, printer, phone nodes laptop · printer · phone links none shared nothing: each device is alone
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.