The idea
A host sending data rarely has a direct wire to the host it is sending to. The packet has to cross a chain of routers, each one only physically connected to its immediate neighbours. Something has to work out which neighbour to hand the packet to at each step, and something else has to guarantee the packet actually gets moved once that decision is made. Those are two different jobs, and the network layer is where the first one lives.
How it works
Hop-to-hop vs end-to-end delivery
The lecture draws this distinction directly:
- End-to-end delivery (also called host-to-host delivery) is a network layer job. It covers the entire journey from the original source to the final destination.
- Hop-to-hop delivery is a data link layer job. It covers exactly one physical link — the frame going from one device to the next device wired or radio-linked to it.
A single end-to-end delivery may contain multiple hops. The lecture illustrates this with a network topology diagram that did not survive extraction from the slides, but the idea does not depend on the picture: the data link layer only ever sees as far as the next device on the wire, while the network layer is responsible for tracking the whole path and deciding, hop by hop, where the packet goes next. This is also why a network layer address has to be globally meaningful across the whole internetwork, while a link layer address only ever has to be meaningful on the one link it is used on.
How it works
Datagram networks
Network layer function. The network layer exists to solve two problems: host-to-host delivery, and routing packets over multiple networks.
Addressing scheme. Two addresses coexist for a reason: each device has its own physical address (the MAC address), but a logical IP address is what uniquely identifies a host across the whole internetwork. The MAC address only has to be unique on its own link; the IP address has to be unique globally.
Service provided. A datagram network gives you four things:
- Connectionless — no setup phase and no teardown phase.
- Best-effort delivery — no service guarantee, which means no flow control and no error control.
- Unreliable — it makes no attempt to recover from a failure itself.
- But fast — all of the above is what buys the speed.
Internet model. Three consequences follow from treating the network as a pure datagram network: there is no call setup at the network layer, packets are routed using the destination host’s address rather than any pre-arranged circuit, and packets travelling between the same source-destination pair may take different paths from one another.
How it works
Introduction to routing
Definition. Routing is the act of moving information across an internetwork from a source to a destination. It occurs at Layer 3, the network layer.
Basic activities. Routing is really two activities bundled under one name:
- Determining optimal routing paths. This is the complex part — deciding which path a packet should take.
- Transporting packets through an internetwork. This is packet switching, and it is comparatively straightforward once a path has been decided.
Contrast with bridging. Routing is often contrasted with bridging. Bridging occurs at Layer 2 of the OSI reference model; routing occurs at Layer 3. They solve superficially similar problems — get this unit of data to where it needs to go — at different layers, with different address types and a different scope.
| Bridging | Routing | |
|---|---|---|
| Layer | Layer 2 (data link) | Layer 3 (network) |
| Address used | MAC address | IP address |
| Scope | Within one broadcast domain / link | Across an internetwork, between different networks |
How it works
Routing path determination
Routing protocols use various mechanisms to evaluate which path is best for a packet. To do this, routing algorithms initialise and maintain routing tables, which hold route information.
Routers talk to one another and keep their routing tables current. The routing update message is how they do it — generally all or part of one router’s routing table, sent to its neighbours. By analysing the routing updates it receives from every other router it hears from, a router builds up a detailed picture of the network’s topology.
Check yourself
- End-to-end delivery is the network layer’s job; hop-to-hop delivery is the data link layer’s job, and one end-to-end delivery can span several hops.
- A datagram network is connectionless, best-effort, and unreliable, but fast — those four properties are a package, not a menu.
- Routing has two activities: determining the path (complex) and transporting packets along it (packet switching, straightforward).
- Bridging is Layer 2; routing is Layer 3. Same general goal, different layer.
- A routing table is built from routing update messages exchanged between routers.
In the exam
- State the hop-to-hop / end-to-end distinction precisely, including which layer does which. This is a common short-answer question and the layer names are worth marks in themselves.
- Give both basic activities of routing, not just “finding the best path” — transporting packets through the internetwork is the second, and examiners sometimes ask for both by name.
- Know the four service properties of a datagram network as a set: connectionless, best-effort, unreliable, fast. Missing one from a list question is a common way to lose a mark here.
- Bridging vs routing is a layer question first. If asked to distinguish them, lead with Layer 2 vs Layer 3, then the address type.