What a network is

A network is nodes joined by links so that hosts can exchange data. Everything travels as bits carried by signals (voltage, light, radio); meaning comes from an agreed encoding.

Links are point-to-point (two ends) or broadcast (everyone hears). Directions: simplex, half-duplex, full-duplex. Topologies: mesh n(n − 1)/2 links, star n, ring n, tree n − 1, bus one backbone. LAN ⊂ WAN ⊂ the Internet. In the middle: hub (repeat to all), switch (deliver inside a LAN), router (pick the next network).

From 0.1.1 Why networks exist: hosts, links and sending data

• Network = devices + links that exchange data.
• Host = where a message starts or ends (laptop, phone, server).
• Link = physical path: copper (voltage), fibre (light), wireless (radio).
• All content → bits → signals → bits → content; meaning comes from an agreed encoding.
• One link per pair of n hosts: n(n − 1)/2 links — too many, so links are shared.

From 0.1.2 Nodes, links, topologies, LAN/WAN and the boxes in the middle

• Node = any device; host = node where messages start/end.
• Point-to-point (2 nodes) vs broadcast (all hear all).
• Simplex (one way) · half-duplex (two ways, one at a time) · full-duplex (both at once).
• Links: mesh n(n−1)/2, star n, ring n, tree n−1, bus 1 backbone.
• LAN (building) ⊂ WAN (cities) ⊂ the Internet (network of networks).
• Hub repeats to all · switch delivers within a LAN · router picks the next network.

1. NAT · 1 mark

   A full mesh has 66 links. How many nodes does it have?

   Write your answer before opening the key.

2. MCQ · 1 mark

   Which is NOT a host?

   1. A video server
   2. A tablet
   3. A hub
   4. A laptop running a browser
3. MSQ · 1 mark

   Which statements about a star of n outer nodes are correct?

   1. It needs n links.
   2. A single outer link failure cuts off only one node.
   3. Failure of the centre cuts off every node.
   4. It needs n(n − 1)/2 links.
4. MCQ · 1 mark

   A walkie-talkie link is:

   1. simplex
   2. half-duplex
   3. full-duplex
   4. a broadcast mesh
5. MCQ · 1 mark

   Two hosts receive every bit correctly but cannot understand each other. Most likely:

   1. The link is broadcast.
   2. They do not share the same encoding rules.
   3. One of them is a router.
   4. The link is full-duplex.
6. NAT · 2 marks

   Plan A: full mesh of 20 hosts. Plan B: star of the same 20 hosts. How many more links does Plan A need than Plan B?

   Write your answer before opening the key.

7. MCQ · 1 mark

   Which box joins a home network to the provider's network and picks the next network to use?

   1. Switch
   2. Hub
   3. Router
   4. Fibre
8. MSQ · 1 mark

   Which are reasons networks share links instead of giving every pair a private link?

   1. Private links grow like n²/2.
   2. Laying cables is expensive.
   3. Shared links remove the need for any rules.
   4. Few links can carry many conversations.