Network architecture

Reliability, transmission speed, scalability, data collisions and cost. No shape wins on all five, so every recommendation is a trade between them.

Every device connects to one central switch, which makes it cheap, easy to extend and nearly collision-free. Its weakness is that the switch is a single point of failure: if it fails, nothing reaches anything.

Every pair of devices has its own link, so there are many routes and no single link matters. But links grow roughly with the square of the devices — 5 need 10, 20 need 190 — so it is reserved for a few critical points.

Stars joined together, usually by more resilient links between them. Cheap where the devices sit, reliable where the traffic concentrates — which is what almost every real office and campus network looks like.

Because the switch sends each frame only to the device it is addressed to, so devices are never competing for one shared line.

It runs software that **waits for requests and responds**. A **role**, not a kind of hardware — one machine can run several.

**One copy** of the data, permissions and backup in one place, shared powerful hardware, and one update reaching everyone.

A **single point of failure** and a bottleneck — answered by **redundant servers behind a load balancer**.

**Client–server** centralises control and consistency. **Peer-to-peer** has no single failure point and **gets faster as peers join**, but security and consistency are harder.

**Up** — a bigger machine; simple, but has a ceiling and is still one failure point. **Out** — more machines; no ceiling, but the software must allow it.

In client-server one machine holds the data and answers requests from the rest. In peer-to-peer there is no machine in charge — every machine holds a share and serves the others directly.

Benefit: one authoritative copy of the data, with security, access and backups controlled in a single place. Drawback: the server is a single point of failure and can bottleneck when many clients want it at once.

Benefit: no single point of failure, and capacity grows as more peers join, with no expensive server. Drawback: no central control over who sees what, data may disagree between peers, and availability depends on peers being online.

Because an account balance must have one correct value held and controlled by the bank. Copies spread across customers' machines could disagree and none would be authoritative.

By design: every participant holds a copy of the ledger and they agree among themselves what it says, so no single organisation decides what is true.

Splitting one network into smaller separate ones. For performance, because less traffic competes on each segment, and for security, because a break-in or infection is contained where it started.

Dividing the IP address range into smaller ranges, each its own network. Traffic between subnets must pass through a router, which is where rules can be enforced.

A logical grouping of devices configured on the switch, regardless of where each device is plugged in. Two machines in one room can be on different VLANs, and moving someone between groups is a setting rather than re-cabling.

Broadcast traffic reaches only its own segment rather than every device, so on a network split six ways each device sees roughly a sixth as much — the cabling carries useful work instead of noise.

It must be designed and maintained: wrong rules block legitimate work and forgotten rules leave gaps. Each crossing between segments also adds a routing step, and so a little delay.