IP Addressing

Every device on the Internet needs a unique IP address so that packets can be routed to the correct destination. At A-Level you must understand IPv4, IPv6, the concept of subnetting, and the difference between public and private addresses.

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IPv4

IPv4 (Internet Protocol version 4) uses a 32-bit address, typically written in dotted-decimal notation — four decimal numbers (octets) separated by dots.

Example:  192.168.1.100

Binary:   11000000.10101000.00000001.01100100

Address Space

• 32 bits → 2^32 = approximately 4.3 billion unique addresses.
• This seemed enormous in the 1980s but is now insufficient for the number of devices connected to the Internet.

Structure: Network Part and Host Part

An IPv4 address is divided into a network part (identifies the network) and a host part (identifies the specific device on that network).

The subnet mask determines which bits belong to the network part and which belong to the host part.

IP Address	Subnet Mask	Network Part	Host Part
192.168.1.100	255.255.255.0	192.168.1	.100
10.0.0.50	255.0.0.0	10	.0.0.50

In the subnet mask, 1-bits mark the network part and 0-bits mark the host part:

• 255.255.255.0 in binary = 11111111.11111111.11111111.00000000 → the first 24 bits are the network, the last 8 are the host.

CIDR Notation

The subnet mask can also be written as a prefix length using CIDR (Classless Inter-Domain Routing) notation:

• 192.168.1.100/24 means the first 24 bits are the network part.
• 10.0.0.50/8 means the first 8 bits are the network part.

Special Addresses

Address	Purpose
127.0.0.1	Loopback (localhost) — a device sending data to itself
0.0.0.0	Default route / unspecified address
255.255.255.255	Broadcast — sent to all devices on the local network

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Public vs Private IP Addresses

Private Addresses

Certain address ranges are reserved for use within private networks (LANs). They are not routable on the public Internet.

Range	CIDR	Typical Use
10.0.0.0 – 10.255.255.255	10.0.0.0/8	Large organisations
172.16.0.0 – 172.31.255.255	172.16.0.0/12	Medium organisations
192.168.0.0 – 192.168.255.255	192.168.0.0/16	Home networks, small businesses

NAT (Network Address Translation)

Because private addresses cannot be used on the Internet, a NAT router translates private addresses to a single (or a few) public IP addresses when traffic leaves the local network. This allows many devices to share one public address.

LAN devices (private IPs) ──→ [NAT Router] ──→ Internet (public IP)
192.168.1.10                   Public: 80.1.2.3
192.168.1.11
192.168.1.12