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A network is more than cables and computers: between the end devices sits a family of specialised hardware whose job is to move data correctly and efficiently from sender to receiver. Each device is designed to operate at a particular layer of the network model and to use a particular kind of address, and understanding which device does what at which layer is the single most exam-critical skill in this topic.
This lesson works through the key devices — the NIC, hub, switch, router, wireless access point, gateway, bridge, repeater and firewall — and grounds them in two unifying ideas. The first is the layered model: physical-layer devices (Layer 1) deal only with raw signals, data-link devices (Layer 2) deal with MAC addresses within a single network, and network-layer devices (Layer 3) deal with IP addresses between networks. The second is the distinction between a MAC address (a permanent hardware identifier baked into a NIC) and an IP address (a logical, changeable address used for routing across networks). Keep returning to "what layer, what address" for every device and the whole topic falls into place.
This lesson addresses the parts of OCR H446 section 1.3.3 concerned with the hardware that builds and connects networks, specifically:
Examiners frequently ask candidates to distinguish two similar devices (hub vs switch, switch vs router, router vs gateway). The marks come from naming the layer, the address type and the forwarding behaviour precisely.
Before the individual devices, fix the map they sit on. Network models (covered fully in the protocols and TCP/IP lessons) divide communication into layers; each device operates at the lowest layer it needs to do its job.
graph TD
L3[Layer 3 — Network: IP addresses\nROUTER]
L2[Layer 2 — Data Link: MAC addresses\nSWITCH, BRIDGE, NIC]
L1[Layer 1 — Physical: raw signals\nHUB, REPEATER, cabling]
L3 --> L2 --> L1
The rule of thumb: the higher the layer a device works at, the more "intelligent" it is — it understands more about the data and can make smarter forwarding decisions, but it also does more processing per packet. A hub blindly repeats electrical signals; a switch reads frame headers; a router reads packet headers and consults a routing table; a gateway may rewrite the data entirely. This hierarchy of intelligence is the thread connecting every device below.
There is a practical reason for this division of labour: doing less work lets a device be faster and cheaper, so each task is handled at the lowest layer that can do it. A repeater that only boosts signals can be a tiny, cheap component; a router that must examine every packet and consult a routing table is a far more complex and expensive machine. By keeping simple jobs (signal regeneration, local frame delivery) on simple devices and reserving the expensive cross-network decisions for routers, a network gets both performance and economy. It also means a fault can often be localised by layer: a physical problem (a dead cable, a failed repeater) looks different from a routing problem (packets reaching the local switch fine but never leaving for the Internet), which is invaluable when troubleshooting. Keep this "lowest layer that can do the job" principle in mind — it explains not just what each device does, but why networks are built from a mixture of simple and intelligent devices rather than one all-powerful box.
Because almost every device decision hinges on addressing, get this contrast straight first.
| Aspect | MAC address | IP address |
|---|---|---|
| Full name | Media Access Control address | Internet Protocol address |
| Assigned by | The manufacturer, burned into the NIC | Configured by software / DHCP |
| Changes? | Permanent (physical) | Can change (logical) |
| Scope | Identifies a device on the local network | Identifies a device across interconnected networks |
| Layer | Data link (Layer 2) | Network (Layer 3) |
| Length | 48 bits (6 hex pairs) | 32 bits (IPv4) or 128 bits (IPv6) |
| Used by | Switches, bridges, NICs | Routers, the Internet |
A helpful analogy: a MAC address is like the serial number on a specific delivery van — fixed, unique, identifying the physical vehicle. An IP address is like the postal address it is currently parked at — it describes where on the network the device sits and can change if the device moves to a different network. Local delivery within one building uses the van's identity (MAC, switch); delivery across the country uses postal addresses (IP, router).
The length of an address determines how many unique devices it can identify, which is a direct application of binary place-value. A MAC address is 48 bits, so the total number of distinct MAC addresses is:
248≈2.8×1014— roughly 281 trillion, comfortably enough to give every NIC ever manufactured a unique value. By contrast an IPv4 address is only 32 bits:
232≈4.3×109— about 4.3 billion, which sounds large but is fewer than the number of people on Earth, and this exhaustion is precisely why IPv6 (128 bits) was introduced, as the TCP/IP-in-detail lesson explains. Two lessons in this unit therefore converge here: the number of bits is a data-representation idea, while which device reads which address is the hardware idea of this lesson.
A Network Interface Card (NIC) is the hardware that physically connects a device to the network medium and gives it its identity on the local network.
| Feature | Detail |
|---|---|
| Function | Provides the physical and data-link interface between a device and the network |
| Types | Wired (Ethernet, RJ45 socket) or wireless (Wi-Fi radio) |
| MAC address | Holds the device's unique 48-bit MAC address, set by the manufacturer |
| Layer | Operates at Layers 1-2 (physical signalling and frame handling) |
| Location | Built into the motherboard or added as an expansion card / USB adapter |
A MAC address is 48 bits, conventionally written as six pairs of hexadecimal digits. The first three pairs identify the manufacturer (the Organisationally Unique Identifier), the last three a unique serial within that manufacturer:
| MAC portion | Example bytes | Meaning |
|---|---|---|
| Manufacturer (OUI) | 00:1A:2B | Organisationally Unique Identifier — identifies the maker |
| Device | 3C:4D:5E | Unique serial assigned by that manufacturer |
The NIC is where a frame is physically turned into electrical, optical or radio signals to put on the wire, and where incoming signals are turned back into frames — so it straddles the boundary between Layer 1 (physical) and Layer 2 (data link).
A hub is the simplest multi-port device: it connects several devices in a physical star but has no intelligence whatsoever.
| Feature | Detail |
|---|---|
| Function | Receives a signal on one port and repeats it out of every other port |
| Layer | Physical (Layer 1) |
| Addressing | None — it does not read addresses at all |
| Collisions | High — all ports share one collision domain |
| Status | Obsolete — superseded by switches |
Because a hub simply broadcasts every incoming frame to all ports regardless of the intended recipient, two problems follow. First, wasted bandwidth and a security risk: every device receives every frame, so a device can eavesdrop on traffic meant for others. Second, collisions: since all devices share a single collision domain, two transmitting at once corrupt each other and must retry — exactly the bus-topology problem from the previous lesson. A hub makes a physical star behave as a logical bus, which is why it was replaced by the switch.
A switch also connects devices in a star, but it is intelligent: it learns where each device is and forwards each frame only to the port that needs it.
| Feature | Detail |
|---|---|
| Function | Forwards frames selectively using MAC addresses |
| Layer | Data link (Layer 2) |
| Addressing | Reads source and destination MAC addresses |
| MAC address table | Builds a table mapping MAC addresses to ports |
| Collisions | Minimal — each port is its own collision domain |
| Performance | High — effectively full bandwidth per port, full-duplex |
A switch is not pre-programmed with where devices are; it learns by observation:
graph TD
A[Frame arrives on a port] --> B[Read SOURCE MAC]
B --> C[Record SOURCE MAC and arrival port in MAC table]
C --> D[Read DESTINATION MAC]
D --> E{Destination in MAC table?}
E -->|Yes| F[Forward only to that single port]
E -->|No| G[Flood to all ports except the source]
Because each port is its own collision domain and the switch forwards selectively, a switched star is a true star — fast, private and collision-free — which is why switches are the backbone of every modern LAN.
A router connects different networks and forwards data between them using IP addresses. Where a switch moves frames within one network, a router moves packets between networks — for example from a home LAN to the Internet.
| Feature | Detail |
|---|---|
| Function | Routes packets between separate networks along the best path |
| Layer | Network (Layer 3) |
| Addressing | Reads source and destination IP addresses |
| Routing table | Holds known networks and the best next hop towards each |
| NAT | Often performs Network Address Translation (many private devices share one public IP) |
| DHCP | Often also acts as a DHCP server, handing out IP addresses |
| Feature | Switch | Router |
|---|---|---|
| Operates at | Layer 2 (MAC) | Layer 3 (IP) |
| Connects | Devices within one network | Different networks together |
| Addressing | MAC addresses | IP addresses |
| Broadcasts | Forwards within the LAN | Blocks broadcasts between networks |
| Decision based on | MAC address table | Routing table |
The home "router" most people own is really a combined device — a router, a switch (the LAN ports), a wireless access point and a firewall in one box — which is why the distinction confuses beginners. Conceptually, though, the routing function is purely Layer 3: read the destination IP, consult the routing table, forward to the best next hop.
The roles of switch and router crystallise when you trace one journey end to end. Suppose PC A in an office wants to load a web page from a server on the Internet:
This single walkthrough captures the heart of the topic: switches forward by MAC within a network; routers forward by IP between networks; and at each hop the frame's MAC addresses change while the packet's IP addresses persist. If you can narrate this journey, you understand network hardware.
A WAP lets Wi-Fi devices join a wired network by bridging radio and cable.
| Feature | Detail |
|---|---|
| Function | Connects wireless devices to the wired LAN |
| Layer | Operates around Layers 1-2 |
| Connection | Plugs into a switch or router via Ethernet |
| Standards | The 802.11 family (e.g. 802.11ac, 802.11ax / Wi-Fi 6) |
| Range | Typically tens of metres indoors |
| Security | WPA2 / WPA3 encryption, optional MAC filtering |
A WAP is essentially a Layer-2 bridge between two media — radio and copper. Note that, like a hub's shared cable, the radio medium is shared among all associated devices, so wireless behaves logically like a bus and needs a collision-avoidance scheme (CSMA/CA), a point developed in the wireless-networking lesson.
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