Build the idea first, then use the next activity to check it.
What is a network?
Two devices, one link, and the problem that addressing exists to solve.
What you will be able to do
- Describe what a network does in one sentence
- Explain why every device on a network needs an address
- Tell the difference between a link and a network
Before this: Nothing. This is the first lesson in the path.
Why it matters: Every fault you will ever troubleshoot is a failure of one of three things: the link, the address, or the path. Naming those three now means every later lesson has somewhere to attach.
A network is two or more devices that can send each other messages. That is the whole idea. Everything else - switches, routers, addresses, protocols - exists to make that work when there are more than two devices and they are not sitting next to each other.
A link is not a network
Connect two computers with a single cable and you have a link. Both machines can send electrical signals to each other, but neither one knows who it is talking to, and neither can talk to a third machine that is not on that cable.
A network is what you get when you add two things to a link: addresses, so a device can say who a message is for, and devices whose job is to move messages between links.
Why addresses
On a link with two devices, "send this to the other one" is unambiguous. Add a third device and it stops being unambiguous, so each device needs a name that the others can put on a message. On the networks you will build here, that name is an IPv4 address - four numbers separated by dots, like 192.168.10.25.
An address does two jobs at once, and this is the part that catches beginners out. It identifies a specific device, and it also says which network that device is on. The next module is entirely about how one number manages to do both.
A message crossing two networks
A laptop on 192.168.1.0/24 wants to reach a server on 10.0.5.0/24. The laptop notices the server is not on its own network, so it hands the message to its router. The router looks at its routing table, finds the network the server is on, and forwards it. Two decisions, made by two different devices, for one message.
Terminology
- Link
- A single physical or virtual connection between two devices. No addressing required.
- Network
- A group of devices that can reach each other directly, identified by a shared address range.
- Host
- Any device that is an endpoint for traffic - a laptop, a server, a phone. It sends and receives, but does not forward for others.
Key takeaways
- A network is devices that can send each other messages; a link is just one connection.
- Addresses exist so a message can say who it is for once there are more than two devices.
- An IPv4 address identifies both a device and the network it sits on.
- Every connectivity fault is a broken link, wrong addressing, or a missing path.
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