Network types, topologies and devices
A network is two or more devices connected so they can share data and resources such as files, printers and an internet connection. Every network, from a home Wi-Fi setup to a hospital campus, can be described three ways: the area it covers, how its devices are arranged (its topology), and which devices move the traffic.
Networks by size
| Type | Covers | Workplace example |
|---|---|---|
| PAN (personal area network) | A few meters around one person | A phone paired with a Bluetooth headset and a smartwatch |
| LAN (local area network) | One building or floor | The wired and Wi-Fi network inside a clinic office |
| WLAN (wireless LAN) | A LAN that uses Wi-Fi | Guest Wi-Fi in a hotel lobby |
| CAN (campus area network) | Several buildings on one site | A university or hospital campus linking its buildings |
| MAN (metropolitan area network) | A city or metro region | A city government linking its offices across town |
| WAN (wide area network) | Regions, countries, the world | A retail chain linking stores in five states; the internet is the largest WAN |
| SAN (storage area network) | A dedicated network for storage | Servers in a data center reaching shared disk arrays over Fibre Channel or iSCSI |
Topologies
Topology is the layout of the connections. The physical topology is how the cables or radio links actually run; the logical topology is how data flows. A network can look like a star physically and still behave like one shared segment logically, which is what happens with an old hub.
| Topology | How it is arranged | Strength | Weakness |
|---|---|---|---|
| Star | Every device connects to a central switch or access point | Easy to add devices and find faults; one bad cable affects one device | The central device is a single point of failure |
| Bus | All devices share one backbone cable | Cheap and simple (legacy) | A break in the backbone stops the whole segment; collisions grow with traffic |
| Ring | Each device connects to two neighbors in a loop | Predictable traffic (legacy Token Ring, some metro fiber rings) | One break can stop the ring unless it is a dual ring |
| Mesh | Devices connect to many or all others (partial or full mesh) | Redundant paths survive link failures; used in WAN cores and wireless mesh | Costly: a full mesh of n devices needs n(n-1)/2 links |
| Hybrid | A mix, such as several stars joined by a mesh core | Fits real sites | More complex to design and document |
| Point-to-point | A direct link between two endpoints | Simple and private | Does not scale on its own |
Star is the answer for almost every modern office LAN. Mesh is the answer when a question stresses redundancy or no single point of failure.
Devices that move traffic
| Device | OSI layer | What it does |
|---|---|---|
| Hub | 1 (Physical) | Repeats every signal out of every port; all devices share the bandwidth and see all traffic. Obsolete, but still tested. |
| Repeater | 1 (Physical) | Regenerates a weak signal to extend a cable run or wireless range |
| Modem | 1 (Physical) | Converts between the provider's signal (cable, DSL, fiber) and Ethernet |
| Bridge | 2 (Data Link) | Joins two LAN segments and filters frames by MAC address |
| Switch | 2 (Data Link) | Learns which MAC address sits on which port and forwards each frame only where it needs to go |
| Wireless access point (WAP) | 2 (Data Link) | Connects Wi-Fi clients to the wired LAN |
| Router | 3 (Network) | Forwards packets between different networks using IP addresses and a routing table; it is the default gateway for a LAN |
| Layer 3 switch | 3 (Network) | A switch that can also route between VLANs inside a building |
| Firewall | 3 to 7 | Allows or blocks traffic by rules; next-generation firewalls also inspect applications |
| Load balancer | 4 or 7 | Spreads requests across several servers so that no single server is overloaded |
| Proxy server | 7 (Application) | Makes requests on behalf of clients, often to filter, log or cache web traffic |
Hub versus switch is a classic trap. A hub sends everything everywhere, so all its ports share one collision domain; a switch sends a frame only to the port of the destination MAC address, so each port is its own collision domain. A router separates broadcast domains; a single switch without VLANs does not.
Client-server and peer-to-peer
In a client-server network, dedicated servers provide services such as file shares, email and directory logins, and clients request them. Businesses work this way because servers can be secured, backed up and managed in one place. In a peer-to-peer network, every computer shares and requests resources directly; that is cheap for two or three home computers but hard to secure and manage at scale.
Word bank
All definitions →- Broadcast domain
- The set of devices that receive each other's broadcasts; routers separate them
- Collision domain
- The part of a network where two devices sending at once can collide; each switch port is its own
- Default gateway
- The router a device sends traffic to when the destination is on another network
- Single point of failure
- One component whose failure stops the whole system
- Topology
- The physical or logical arrangement of a network's connections
Knowledge check
A small dental office wants a wired network where one failed cable affects only one computer and new computers are easy to add. Which topology fits best?
Which device forwards frames only to the port where the destination MAC address was learned?
A company links offices in Denver, Chicago and Atlanta over leased lines. What type of network is this?