Describe the hardware that is used to support a LAN

2.1 Networks – The Internet

Objective

Describe the hardware that is used to support a Local Area Network (LAN) and explain how it connects with the Cambridge IGCSE/A‑Level Computer Science (9618) syllabus. The note also covers the required networking theory – information representation, addressing, subnetting, network services and security – so that students can answer all relevant exam command‑words (describe, explain, justify, compare, evaluate).

1. Syllabus Mapping – What the Exam Expects

2. Information Representation (Relevant to Networking)

• Binary & hexadecimal – computers store all data as bits. Example: 10110110₂ = B6₁₆ = 182₁₀.
• BCD (Binary‑Coded Decimal) – each decimal digit is stored in 4 bits. Example: decimal 59 → 0101 1001.
• ASCII – 7‑bit code for characters. Example: 'A' = 0100 0001₂ = 41₁₆ = 65₁₀.
• Unicode (UTF‑8) – extends ASCII to cover world scripts; uses 1‑4 bytes per character. Example: Euro sign € = E2 82 AC (hex).
• Floating‑point (IEEE‑754 single precision) – 1 sign bit, 8 exponent bits, 23 mantissa bits. Example: 5.75 → 0100 0001 0111 0000 0000 0000 0000 0000.
• Compression

  • Lossless (e.g., Run‑Length Encoding – RLE). AAAABBBCCDAA → 4A3B2C1D2A (size reduced from 12 to 9 characters).
  • Lossy (e.g., JPEG for images) – discards information that is less noticeable to the eye, achieving much higher reduction.

3. Transmission Media

The physical pathway determines speed, distance, cost and susceptibility to interference.

4. Network Interface Devices (NICs)

• Wired Ethernet NIC – RJ‑45 connector; supports 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps (auto‑negotiation of speed/duplex).
• Fiber NIC – Uses SFP (1 Gbps) or SFP+ (10 Gbps) modules; connects to single‑mode or multimode fibre.
• Wireless NIC – Integrated Wi‑Fi adapter with antenna; supports IEEE 802.11a/b/g/n/ac/ax and handles association, authentication (WPA2/WPA3) and encryption.

Common NIC functions:

• Frame creation & parsing (Ethernet MAC header, CRC).
• MAC address assignment – a unique 48‑bit identifier stored in ROM/EEPROM.
• Hardware off‑loading (checksum, TCP segmentation) to reduce CPU load.

5. Network Infrastructure Devices

5.1 Hubs (Obsolete)

• Layer 1 multi‑port repeaters; broadcast incoming frames to all ports.
• Creates a single collision domain – leads to collisions and shared bandwidth.
• Exam point: state that hubs are now largely obsolete and have been replaced by switches.

5.2 Switches

• Layer 2 devices that forward frames based on MAC addresses.
• Each port is a separate collision domain; full‑duplex eliminates collisions.
• Learning – builds a MAC address table from source addresses.
• Forwarding – sends a frame only to the appropriate port (or floods if unknown).

Types of switches:

• Unmanaged – plug‑and‑play, no configuration.
• Managed – VLANs, port security, STP, QoS, SNMP, link aggregation.
• Power over Ethernet (PoE) – supplies 48 V DC to devices such as IP phones, APs or cameras.

5.3 Routers

• Layer 3 devices that connect different LANs or a LAN to a WAN/Internet.
• Maintain a routing table (destination network → outgoing interface).
• Default gateway – router address that hosts use for off‑network traffic.
• Static routing (manual) vs. dynamic routing (RIP, OSPF, EIGRP).
• Common built‑in services: DHCP server, NAT, firewall, and often Wi‑Fi AP (gateway).

5.4 Access Points (APs)

• Bridge the wireless medium to the wired Ethernet backbone.
• Broadcast one or more SSIDs; support WPA2‑PSK, WPA3‑SAE, or enterprise 802.1X.
• Channel selection (auto or manual) to minimise co‑channel interference.
• Typically powered via PoE.

5.5 Modems / Gateways

• Convert the ISP’s signal (DSL, cable, fibre) into Ethernet for the LAN.
• Types:

  • DSL modem – uses telephone lines (ADSL/VDSL).
  • Cable modem – uses coaxial cable (DOCSIS).
  • FTTH ONT – terminates fibre and provides Ethernet.

• Modern “gateway” devices combine modem, router, DHCP, NAT, firewall and often Wi‑Fi AP.

6. LAN Topologies – Physical Layout & Performance

7. Protocol Stack – OSI vs. TCP/IP

Cambridge exams refer to the “protocol stack”. The TCP/IP model is the one actually implemented; the OSI model is useful for understanding responsibilities.

8. Ethernet Frame & MAC Addressing

• Frame structure – Preamble – SFD – Dest‑MAC – Src‑MAC – EtherType/Length – Payload – CRC.
• MAC address – 48‑bit (sometimes 64‑bit) globally unique identifier, e.g. 00‑1A‑2B‑3C‑4D‑5E.
• Speeds: 10BASE‑T (10 Mbps), 100BASE‑TX (Fast Ethernet), 1000BASE‑T (Gigabit), 10GBASE‑T (10 Gbps).
• CSMA/CD is only relevant for half‑duplex Ethernet; modern switched LANs use full‑duplex, so collisions are eliminated.

9. IP Addressing, Subnetting, NAT & DHCP

9.1 IPv4 Address Structure

• 32‑bit address written as four decimal octets: 192.168.1.10.
• Subnet mask separates network and host portions. Example: 255.255.255.0 (= /24).

9.2 Private vs. Public IP Ranges (exam‑required list)

9.3 Subnetting Example (Class C /24 LAN)

Network: 192.168.1.0/24
Goal: 4 sub‑nets of equal size.
Step 1 – Borrow 2 bits from the host part (2² = 4 subnets):
New mask = 255.255.255.192  → /26
Subnets:
1. 192.168.1.0   – 192.168.1.63   (hosts 1‑62, broadcast .63)
2. 192.168.1.64  – 192.168.1.127  (hosts 65‑126, broadcast .127)
3. 192.168.1.128 – 192.168.1.191  (hosts 129‑190, broadcast .191)
4. 192.168.1.192 – 192.168.1.255  (hosts 193‑254, broadcast .255)
Each subnet has 62 usable host addresses (2⁶‑2).

9.4 Network Address Translation (NAT)

• Maps many private IPv4 addresses to a single public address.
• Provides a basic firewall – inbound packets must match an existing outbound session.
• Exam point: NAT conserves IPv4 address space and adds a layer of security.

9.5 Dynamic Host Configuration Protocol (DHCP)

• Automatically assigns IP address, subnet mask, default gateway, DNS server.
• Typical lease time: 24 h (can be configured).
• Routers or dedicated DHCP servers commonly provide the service in a LAN.

10. DNS & URL – How Names Become Addresses

• DNS – hierarchical, distributed database that maps domain names to IP addresses (e.g., www.example.com → 93.184.216.34).
• URL structure – protocol://hostname[:port]/path?query#fragment.

  
  Example: https://www.example.com:443/articles?id=7#section2.

• In a LAN, a local DNS server (often part of the router) can resolve internal hostnames (e.g., printer1.local).

11. Wireless Standards & LAN Security

11.1 Wi‑Fi Standards (IEEE 802.11)

11.2 Security Mechanisms

• Encryption – WPA2‑PSK (AES) or WPA3‑SAE (more resistant to password‑guessing).
• Authentication – Pre‑Shared Key (PSK) for homes; 802.1X/EAP for enterprises.
• MAC‑address filtering – Switch or AP permits only listed MACs.
• VLANs (Virtual LANs) – Separate broadcast domains on a single physical switch; improve security and reduce unnecessary traffic.
• Port security – Limits the number of MAC addresses per switch port, can shut down a port on violation.
• Firewalls – State‑ful inspection on the router/gateway; blocks unsolicited inbound traffic.

12. Exam‑Style Practice (Command‑Word Focus)

1. Describe the role of a switch in a LAN and explain why it is preferred over a hub.
2. Explain how NAT allows many devices on a LAN to share a single public IP address.
3. Justify the choice of Cat 6 twisted‑pair cable for a new office LAN that requires 1 Gbps connections up to 80 m.
4. Compare the advantages and disadvantages of a star topology versus a mesh topology for a university computer‑lab network.
5. Calculate the number of usable host addresses in a 192.168.10.0/27 subnet and list the first and last usable IPs.

Answers should use appropriate terminology (e.g., collision domain, broadcast domain, MAC address table, DHCP lease, VLAN ID) and, where required, include a small diagram or a short calculation.

13. Quick Reference Summary

• Media – UTP (Cat 5e/6), fibre, Wi‑Fi (802.11ax).
• NICs – wired, fibre, wireless; provide MAC address.
• Infrastructure – Switch (Layer 2), Router (Layer 3), AP, Modem/Gateway.
• Topologies – Star (most common), Mesh (high reliability), Hybrid (scalable).
• Protocol stack – Link (Ethernet/Wi‑Fi), Internet (IP), Transport (TCP/UDP), Application.
• Addressing – IPv4 private ranges, subnetting, NAT, DHCP.
• Security – WPA2/WPA3, VLANs, port security, MAC filtering, firewall.
• Information representation – binary/hex, ASCII/Unicode, floating‑point, compression.