An IP address is divided into two primary components:

• Network Address : Identifies the specific network.
• Host Address : Identifies the individual device within that network.

Classful Addressing

In the early days of networking, IP addresses followed a classful addressing system. Each IPv4 address is 32 bits long and typically displayed in dotted decimal format. Each octet (group of 8 bits) ranges from 0 to 255.

Organisations could purchase one of the following IP address classes:

• Class A Network Prefix: 8 bits Example: 44.0.0.1 Network: 44 Host: 0.0.1
• Class B Network Prefix: 16 bits Example: 128.16.0.2 Network: 128.16 Host: 0.2
• Class C Network Prefix: 24 bits Example: 192.168.1.100 Network: 192.168.1 Host: 100
• Class D Network Prefix: 0 bits (since Class D is used for multicast, there’s no fixed division between network and host) Example: 233.0.0.1 Usage: Multicast address used to send data to multiple recipients. Address Range: 224.0.0.0 to 239.255.255.255 Network/Host Split: Class D does not have a traditional network/host structure. It’s designed for group communication where the address refers to a group of receivers rather than a single host.

In simple terms, this fixed structure led to inefficient IP address utilisation, especially for networks that didn’t need the full range offered by each class.

Classless Addressing (CIDR)

Classless Inter-Domain Routing (CIDR) was introduced to solve the limitations of classful addressing. CIDR uses Variable Length Subnet Masking (VLSM) to provide more granular control over how IP address space is divided between network and host portions.

CIDR notation includes an IP address followed by a slash (/) and a number indicating the number of bits used for the network portion.

• Example : 192.0.2.0/24
• Network Prefix : First 24 bits (192.0.2)
• Remaining Bits : Host addresses

A subnet mask is applied to identify the network portion of an IP address by setting the host bits to zero. With VLSM, administrators can create subnets of various sizes, each with a specific number of usable IP addresses, leading to better address space efficiency.

To gain a deeper understanding of IPv4 and IPv6 CIDR structures in AWS, you can refer to the following URL:

https://aws.amazon.com/what-is/cidr/

While having detailed knowledge of these concepts is not mandatory for configuring a VPC, it’s essential to have a basic understanding of IP addresses and subnets for effective network setup.