Describe the role of a router in a network

Computer Science (IGCSE) – Network Hardware: Routers

Learning Objective

Students will be able to describe the role of a router in a network, explain how it forwards packets, and evaluate its advantages and limitations. (AO1 – knowledge, AO2 – application, AO3 – evaluation)

1. Where Routers Fit in the Cambridge IGCSE Computer Systems Syllabus

The router topic sits within Topic 3 – Hardware and links to Topic 5 – Internet & WWW. The table shows the key syllabus points that must be covered and the assessment objectives they address.

2. What Is a Router?

• Definition: A router is a networking device that forwards data packets between different computer networks, forming an internetwork.
• OSI layer: Operates at the Network layer (Layer 3) using IP addresses.
• Other names: gateway, default gateway (when it provides the exit point to external networks).

3. Core Functions of a Router (AO2)

• Packet forwarding – examines the destination IP address and sends the packet out the appropriate interface.
• Path selection – chooses the best route using a routing table and routing algorithms.
• Network segmentation – creates separate broadcast domains, reducing congestion.
• Inter‑network connectivity – links LANs to WANs, the Internet, or other LANs.
• Address translation – performs NAT/PAT so many devices can share a single public IP.
• Basic security – implements ACLs, simple firewall rules, and NAT filtering.

4. How a Router Forwards a Packet – Step‑by‑Step (AO2)

1. Receive the frame on an incoming interface (e.g., Ethernet port).
2. Read the destination IP address from the packet header.
3. Lookup the routing table to find the longest‑prefix match.
4. Determine the next‑hop IP address and the outgoing interface.
5. Encapsulate the packet in a new frame suitable for the outgoing media (Ethernet, PPP, etc.).
6. Transmit the frame toward the next hop or final destination.

5. Example Routing Table (AO1)

6. Routing Protocols (AO2)

• Static routing – routes entered manually; simple and predictable but not scalable.
• Dynamic routing protocols:
  • Distance‑vector – e.g., RIP. Uses hop count as metric; updates every 30 s.
  • Link‑state – e.g., OSPF. Each router shares a map of the network; fast convergence.

7. IP Addressing & Subnetting Basics (AO1, AO2)

• IP address format: four octets (32 bits), e.g., 192.168.1.10.
• Subnet mask: separates network and host portions, e.g., 255.255.255.0 (= /24).
• Default gateway: the router’s LAN IP address that hosts send packets to when the destination is outside their own network.

8. NAT and Basic Firewalling (AO2, AO3)

• Network Address Translation (NAT) – rewrites source IP addresses of outbound packets from private to public addresses.
• Port‑address translation (PAT) – also called “NAT overload”; distinguishes internal devices by using different source ports.
• Simple ACL example: deny any → 192.168.1.0/24 from 203.0.113.0/24 blocks inbound Internet traffic to the internal LAN.

9. Advantages and Disadvantages of Using Routers (AO3)

10. Example Network Diagram (Suggested Illustration)

Two LANs are linked by a three‑port router:

• LAN A – 192.168.1.0/24, switch, PCs.
• LAN B – 10.0.0.0/24, switch, PCs.
• Router interfaces:
  • LAN A port – 192.168.1.1
  • LAN B port – 10.0.0.1
  • WAN port – public IP 203.0.113.5 (connected to the Internet)
• All PCs use the router’s LAN IP as their default gateway.

Packet flow example: PC in LAN A → router (looks up 10.0.0.0/24) → forwards out LAN B interface → PC in LAN B.

11. Glossary of Key Terms (AO1)

• IP address – 32‑bit numeric label that uniquely identifies a device on an IP network.
• Subnet mask – Binary mask that separates network and host portions of an IP address.
• Default gateway – Router address used by a host when the destination is outside its own network.
• Routing table – List stored in a router that maps destination networks to next‑hop addresses and interfaces.
• NAT / PAT – Techniques that translate private IP addresses to public ones (PAT also uses port numbers).
• ACL (Access‑Control List) – Set of rules that permit or deny traffic based on IP address, protocol, or port.
• Static route – Manually configured entry in a routing table.
• Dynamic route – Learned automatically via a routing protocol.
• Distance‑vector protocol – Advertises the distance (hop count) to each network (e.g., RIP).
• Link‑state protocol – Shares the state of each link with all routers (e.g., OSPF).

12. Command‑Word Guide & Exam Technique (AO1‑AO3)

Tip for router questions: start with a brief definition (AO1), then describe the forwarding process (AO2), and finish with an evaluation of benefits/limitations (AO3). Use diagrams or tables to save words.

13. Quick Revision Checklist

• Identify the OSI layer a router operates in (Layer 3) – AO1.
• List at least five core router functions – AO1.
• Explain the five‑step packet‑forwarding process – AO2.
• Read and interpret a simple routing table – AO2.
• Distinguish static vs dynamic routing; name one distance‑vector (RIP) and one link‑state (OSPF) protocol – AO2.
• State the purpose of NAT and give a basic example – AO2.
• Evaluate two advantages and two disadvantages of using a router in a small school network – AO3.

14. Action‑Oriented Review Checklist (Alignment with Cambridge 0478)

Use this checklist before the exam to ensure every required point is present and clearly labelled.