Understand the need for NICs and MAC addresses

Network Hardware

Objective

Students will be able to:

• Explain why a computer needs a Network Interface Card (NIC) and link its functions to the OSI model.
• Describe the purpose, structure and variations of MAC addresses, and how switches use them.
• Explain the purpose of IP addresses, the difference between IPv4 and IPv6, subnetting, private vs. public ranges, and the role of ARP and DHCP.
• Describe the role of a router (OSI Layer 3), the concept of a default gateway, and basic routing functions such as NAT and routing tables.

All points are mapped to Cambridge IGCSE 0478 – Topic 3.4 (Network hardware).

1. Network Interface Card (NIC)

A NIC is the hardware that gives a computer a physical link to a network and performs essential data‑link tasks.

Functions and OSI mapping

• Physical layer (OSI 1): Provides the electrical (Ethernet) or radio (Wi‑Fi) interface needed to transmit and receive signals.
• Data‑link layer (OSI 2 – MAC sub‑layer):
  • Frames data into an Ethernet frame (adds MAC header, payload and CRC trailer).
  • Controls access to the shared medium (CSMA/CD for wired, CSMA/CA for Wi‑Fi).
  • Stores a unique Media Access Control (MAC) address that identifies the NIC on the local segment.
  • Can off‑load checksum and CRC calculations, reducing CPU load.
• Form factor: Integrated on most motherboards, or added as a PCI‑e card, USB adapter, or wireless module.

Why a NIC is essential

1. Without a NIC a computer cannot generate or detect the electrical/radio signals required for network communication.
2. The NIC converts internal data into Ethernet frames that obey the network protocol.
3. It supplies the MAC address – a hardware identifier that other devices use to address frames on the local network.
4. By handling framing, error detection and media access, the NIC frees the CPU to run applications.

2. MAC Address – purpose, structure and variations

Purpose

• Provides a globally unique hardware identifier for a NIC.
• Used by switches and other Layer 2 devices to deliver Ethernet frames to the correct destination.

Length and notation

• Most NICs use a 48‑bit address (6 octets). Written as six hexadecimal pairs, e.g. AA:BB:CC:DD:EE:FF.
• Some newer devices (e.g., certain industrial equipment) use a 64‑bit MAC address (8 octets). The notation is similar, e.g. AA:BB:CC:DD:EE:FF:GG:HH.

Structure of a 48‑bit MAC address

• OUI (Organisationally Unique Identifier) – first 3 octets: Identifies the manufacturer (assigned by the IEEE).
• Device identifier – last 3 octets: Assigned by the manufacturer to ensure global uniqueness.

Binary conversion example

MAC: 00:1A:2B:3C:4D:5E
00 = 00000000
1A = 00011010
2B = 00101011
3C = 00111100
4D = 01001101
5E = 01011110

Binary MAC: 00000000 00011010 00101011 00111100 01001101 01011110

OUI = 00:1A:2B → identifies the manufacturer.

How switches use MAC addresses

1. The source NIC places its own MAC address in the frame header and the destination MAC address of the intended recipient.
2. A switch reads the destination MAC, looks it up in its MAC‑address table, and forwards the frame only to the matching port.
3. If the destination MAC is unknown, the switch floods the frame to all ports (except the incoming port). The receiving NIC discards frames that do not match its own MAC.

Learning, ageing and security

• Learning: When a switch receives a frame, it records the source MAC address and the port on which it arrived, building the MAC‑address table dynamically.
• Ageing: Entries that have not been seen for a configurable period (e.g., 300 seconds) are removed to free space and adapt to topology changes.
• MAC‑address spoofing: An operating system can present a different MAC address to the network. The physical NIC still retains its factory‑programmed address, but spoofing can be used for privacy or malicious purposes.

3. IP Address – purpose, types and mapping

Why an IP address is needed

• Provides a logical, network‑layer (OSI 3) identifier that can be changed without altering hardware.
• Allows devices on different LANs or sub‑nets to locate each other and exchange data.

IPv4 vs. IPv6

Version	Length	Notation	Example
IPv4	32 bits	Four decimal octets separated by dots	192.168.1.10
IPv6	128 bits	Eight groups of four hexadecimal digits separated by colons	2001:0db8:85a3:0000:0000:8a2e:0370:7334

Subnet mask / CIDR notation

• A subnet mask separates the network part from the host part of an IPv4 address.
• Common notation: /24 = 255.255.255.0 (24 network bits, 8 host bits).
• Example: 192.168.1.0/24 defines the address range 192.168.1.1 – 192.168.1.254.

Private vs. public IPv4 ranges

Range	CIDR	Typical use
10.0.0.0 – 10.255.255.255	10.0.0.0/8	Large organisations, data‑centres
172.16.0.0 – 172.31.255.255	172.16.0.0/12	Medium‑size networks
192.168.0.0 – 192.168.255.255	192.168.0.0/16	Home and small‑office networks

Static vs. dynamic allocation

• Static (manual): The IP address is entered by the user or administrator and does not change unless re‑configured.
• Dynamic: A DHCP (Dynamic Host Configuration Protocol) server automatically assigns an available IP address each time a device joins the network, and also supplies other parameters such as subnet mask, default gateway and DNS server.

Mapping IP to MAC – ARP

1. The sender knows the destination IP but not the MAC address.
2. It broadcasts an ARP request: “Who has IP 192.168.1.20? Tell 192.168.1.10.”
3. The device with that IP replies with its MAC address.
4. The requester stores the IP‑to‑MAC mapping in its ARP cache for future use (usually a few minutes).

4. Router – role in a network

Key characteristics

• Operates at the network layer (OSI 3) and forwards packets between different LANs or between a LAN and the Internet.
• Uses a routing table to decide the best outgoing interface for each destination IP address.
• Provides the default gateway address that end‑devices use when the destination is outside their own subnet.
• Performs Network Address Translation (NAT) in most home routers, allowing many private IP addresses to share a single public IP.
• Often includes additional services:
  • DHCP server – automatically assigns IP addresses and network parameters.
  • DNS forwarding – relays DNS queries to an upstream DNS server.
  • Basic firewall filtering – blocks unwanted inbound traffic.

Simple routing example

LAN A: 192.168.1.0/24   ←→ Router (eth0 = 192.168.1.1)
LAN B: 192.168.2.0/24   ←→ Router (eth1 = 192.168.2.1)

PC A (192.168.1.10) wants to send data to a printer in LAN B (192.168.2.20):

1. IP layer creates a packet addressed to 192.168.2.20.
2. ARP resolves the MAC address of the default gateway (192.168.1.1).
3. NIC builds an Ethernet frame with destination MAC = router’s MAC.
4. The router receives the frame, strips the Ethernet header, examines the IP header, and forwards the packet out eth1 after performing ARP for the printer’s MAC.
5. The printer replies using the same process in reverse.

Routing concepts (optional depth for advanced learners)

• Static routing: Administrator manually enters routes into the routing table.
• Dynamic routing protocols (e.g., RIP, OSPF) automatically exchange route information. Not required for IGCSE but useful background.

5. End‑to‑end communication flow (simplified)

1. Application creates data → Transport layer (TCP/UDP) adds its header.
2. Network layer adds source and destination IP addresses → IP packet.
3. If the destination is on a different subnet, the packet is sent to the default gateway.
4. ARP (if needed) resolves the gateway’s IP to a MAC address.
5. NIC builds an Ethernet frame: [Dest MAC][Src MAC][Type][Payload][CRC].
6. Switches forward the frame based on the destination MAC; routers replace the Ethernet header when moving the packet between subnets.
7. At the receiver, the process is reversed: Ethernet header removed → IP header examined → transport header processed → application data delivered.

6. Example tables

MAC‑address table (typical switch)

Port	Device	MAC Address (hex)	OUI (Manufacturer)
1	Desktop PC	00:1A:2B:3C:4D:5E	Intel
2	Laptop (Wi‑Fi)	3C:5A:37:9F:12:8B	Qualcomm Atheros
3	Network Printer	AC:DE:48:77:99:00	Realtek
4	Router (LAN side)	00:0C:29:4F:AA:BB	VMware

IP address allocation example

Device	IP Address	Allocation	Notes
Desktop PC	192.168.1.10	Static	Server for classroom resources
Laptop	192.168.1.23	DHCP	Obtained from router
Printer	192.168.1.50	Static (reserved)	Reserved in DHCP pool
Router (WAN)	203.0.113.5	Static (ISP‑provided)	Public address

7. Common misconceptions

• MAC vs. IP address: MAC addresses work only on the local segment (Layer 2) and never change; IP addresses work across networks (Layer 3) and can be reassigned.
• Changing a MAC address: “MAC spoofing” changes the address reported by the operating system, but the physical NIC still contains its factory‑programmed address.
• Uniqueness: The OUI guarantees that manufacturers allocate unique device‑specific portions; duplicate MACs cause collisions and network errors.
• Routers vs. switches: Switches operate at Layer 2 using MAC addresses; routers operate at Layer 3 using IP addresses and the default gateway concept.
• Subnet masks: A subnet mask is not optional – it tells a device which part of the IP address is the network and which part is the host.

8. Summary

A Network Interface Card is the essential hardware that connects a computer to a network, handling both physical‑layer signalling and data‑link‑layer framing. Each NIC carries a unique MAC address (48‑bit or, rarely, 64‑bit) that switches use to forward Ethernet frames. IP addresses provide a logical, routable identifier; IPv4 and IPv6 differ in size and notation, and subnet masks (or CIDR) define the size of the network. Private IPv4 ranges are used inside homes and schools, while DHCP automatically supplies IP configuration. Routers operate at OSI Layer 3, use routing tables and a default gateway to move packets between different networks, and often provide NAT, DHCP, DNS forwarding and basic firewall services. Understanding NICs, MAC addresses, IP addressing and routing forms the foundation for all further study of networking and security in the Cambridge IGCSE curriculum.