What is a Router?

1. What is a Router?

A router is a networking device that forwards data packets between
different networks, directing traffic so that data reaches the correct
destination.

Router = A device that connects multiple networks together and uses
IP addresses to route data packets to their correct destination.

A router operates at the Network Layer using IP addresses.

Every home and office network has at least one router.

The router connects the local network (LAN) to the internet (WAN).

It decides the best path for data to travel across networks.

2. What Does a Router Do?

Key Functions of a Router

Function	Description
Packet forwarding	Reads destination IP and forwards packets toward their destination
Routing	Determines the best path for data to travel across networks
NAT	Translates private IP addresses to the public IP and back
DHCP	Assigns IP addresses automatically to devices on the local network
Firewall	Filters incoming and outgoing traffic for basic security
DNS forwarding	Forwards domain name lookup requests to a DNS server
Wireless access	Modern routers include Wi-Fi capability for wireless devices

3. Router vs Switch vs Hub

A very common exam topic — knowing the difference between these three devices:

Feature	Hub	Switch	Router
Address used	None	MAC Address	IP Address
Sends data to	All devices (broadcast)	Correct device only	Correct network
Works on	Single LAN	Single LAN	Multiple networks
Connects networks?	❌ No	❌ No	✅ Yes
Intelligence	None	Medium	High
Layer	Physical (1)	Data Link (2)	Network (3)

4. The Routing Table

A router uses a routing table to decide where to send each packet.

Routing Table = A database stored in the router that lists known
networks and the best path (next hop) to reach each one.

Example Routing Table

Destination Network	Subnet Mask	Next Hop	Interface
192.168.1.0	255.255.255.0	Direct	LAN port
10.0.0.0	255.0.0.0	192.168.1.254	WAN port
0.0.0.0	0.0.0.0	82.45.67.1	WAN (default)

The 0.0.0.0 default route (also called the default gateway) is used
for any destination not found in the table — traffic is forwarded to the ISP.

5. How a Router Connects LAN to the Internet

Devices on the LAN have private IP addresses (e.g. 192.168.x.x).

The router has two IP addresses:

LAN side (private): e.g. 192.168.1.1 — the default gateway for devices.

WAN side (public): e.g. 82.45.67.23 — assigned by the ISP.

The router uses NAT to translate between private and public addresses.

6. NAT — Network Address Translation

NAT = A technique used by routers to map multiple devices with private
IP addresses to a single public IP address for communication
over the internet.

All outgoing traffic appears to come from the single public IP.

The router keeps track of which private device made each request
so that incoming replies are forwarded back to the correct device.

NAT conserves IPv4 addresses — an entire network can share just one.

7. DHCP — Dynamic Host Configuration Protocol

Most home routers also act as a DHCP server:

DHCP = A service that automatically assigns IP addresses and other
network configuration to devices when they join a network.

Without DHCP, network addresses would have to be manually configured
on every device.

DHCP makes connecting to a network automatic and seamless.

8. Default Gateway

Default Gateway = The IP address of the router on the local network —
the device that data is sent to when the destination is outside the LAN.

Every device on a network is configured with a default gateway address.

When a device needs to reach a destination outside its own network,
it sends the packet to the default gateway (router).

The router then handles getting the packet to its final destination.

9. Packet Routing Across the Internet

Data travelling across the internet passes through many routers,
each one forwarding the packet one step closer to the destination:

Each router reads the destination IP address and decides the
next hop toward the destination.

This process continues until the packet reaches the correct network.

Routers dynamically adapt — if a path is slow or broken,
traffic is rerouted automatically.

10. Wired and Wireless Routers

Modern home routers combine several functions in one device:

11. Router Security Features

Security Feature	Description
Firewall	Blocks unauthorised incoming connections
NAT	Hides internal private IP addresses from the internet
MAC filtering	Only allows specific MAC addresses to connect
WPA2/WPA3 encryption	Encrypts Wi-Fi traffic
Port forwarding control	Controls which external ports are accessible
Admin password	Protects router settings from unauthorised changes

12. Comparison — Router in a Network

13. Key Vocabulary

Term	Definition
Router	A device that forwards data packets between different networks using IP addresses
Routing Table	A database in the router listing known networks and the best path to reach them
Default Gateway	The router's LAN IP address — where devices send traffic destined outside the LAN
NAT	Network Address Translation — maps private IPs to one public IP
DHCP	Dynamic Host Configuration Protocol — automatically assigns IP addresses
Packet	A unit of data transmitted across a network
Next Hop	The next router a packet is forwarded to on its path to the destination
WAN	Wide Area Network — the network outside the LAN, e.g. the internet
LAN	Local Area Network — the private network inside a home or office
Firewall	A security system that monitors and filters network traffic
ISP	Internet Service Provider — the company that provides internet access
DNS	Domain Name System — translates domain names to IP addresses

14. Exam Tips ✅

The most important fact: a router uses IP addresses to route data
between different networks — a switch uses MAC addresses
within a single LAN. This distinction is tested constantly.

Know the two IP addresses a router has:
a private LAN-side address and a public WAN-side address.

NAT allows all devices on a LAN to share one public IP address —
explain this clearly in exam answers.

The default gateway is the router's LAN IP address — devices send
all external traffic here first.

DHCP is usually provided by the router on home networks —
it automatically assigns IP addresses to devices.

A common exam question: "What is the purpose of a router?"
→ To forward data packets between different networks using IP addresses,
connecting the local network to the internet.

Know that modern routers combine router + switch + WAP + DHCP +
NAT + firewall in a single device.

Routing tables allow routers to decide the best path for packets —
mention this for higher-mark questions.

Cambridge (CIE) IGCSE Computer Science — 0478 / 0984

1. What is a Router?#

2. What Does a Router Do?#

Key Functions of a Router#

3. Router vs Switch vs Hub#

4. The Routing Table#

Example Routing Table#

5. How a Router Connects LAN to the Internet#

6. NAT — Network Address Translation#

7. DHCP — Dynamic Host Configuration Protocol#

8. Default Gateway#

9. Packet Routing Across the Internet#

10. Wired and Wireless Routers#

11. Router Security Features#

12. Comparison — Router in a Network#

13. Key Vocabulary#

14. Exam Tips ✅#