What a network is

A computer network lets separate devices exchange data. Phones, laptops, servers, printers, and sensors are endpoints. They are where data starts and where it ends up. Switches and routers are intermediary devices that move data between them without being the final destination.

A device connects through a network interface. An interface may use an Ethernet cable, Wi-Fi radio, virtual adapter, or another link technology. One computer can have several interfaces and several addresses at the same time.

You can see your own interfaces right now. On Linux:

ip -brief link

lo               UNKNOWN        00:00:00:00:00:00
eth0             UP             3c:22:fb:9a:12:40
wlan0            DOWN           a4:83:e7:2c:88:01

lo is the loopback interface, which only talks to the machine itself. eth0 is a wired Ethernet interface, currently UP. wlan0 is a Wi-Fi interface that’s not connected. Each hardware interface has its own link-layer address.

On macOS, run ifconfig and look for en0, which is usually your main interface. The status: active line tells you it’s connected.

Links join interfaces

A link connects interfaces that can communicate directly, with no router in between. Your laptop’s Wi-Fi interface and your home router’s Wi-Fi interface share one link. A network can contain many links joined by routers.

The Internet is the largest example: many independently operated networks agreeing to exchange IP packets. Your home network is one network. Your Internet provider runs another. The server hosting this page sits in yet another. Data crosses all of them.

You can prove your machine knows about at least one other device — the router that connects your link to everything else:

ip route | grep default
# default via 192.168.1.1 dev eth0

That output says: “anything I can’t deliver locally goes to the device at 192.168.1.1, through my eth0 interface.” That device is a router, your first intermediary.

Why this vocabulary matters

Start every network diagram with devices, interfaces, and links. When something breaks, “the network is down” tells you nothing. “My eth0 interface has no address” or “the link to the router is down” points at one specific thing to fix.

Protocol names make more sense once you know which two things are communicating at each step. The rest of this course builds on exactly these three words: endpoint, interface, link.