Understanding IP Addresses, Classful Addressing, and CIDR

1. What is an IP Address?

An IP address (Internet Protocol address) is a unique identifier assigned to a device on a network. It allows devices to locate and communicate with each other.

The most common format (IPv4) looks like this:

Two Key Parts of an IP Address

Every IP address consists of two parts:

• Network portion – identifies the network
• Host portion – identifies a device within that network

The key concept is that a device must be on the same network as another device to communicate with it directly.

For example, if a device is on the network 192.168.250.X, it can communicate directly with other devices on 192.168.250.X. However, it cannot directly communicate with devices on networks such as 192.168.251.X or 192.0.250.X.

The boundary between the network and host portions of an IP address is defined by the subnet mask.

2. Subnet Masks

A subnet mask determines which part of the IP address refers to the network and which part refers to the host.

Example:

This means:

• 192.168.1 → Network
• .10 → Host

3. Classful Addressing

Before modern networking, IP addresses were divided into fixed classes. This is known as classful addressing.

The Main Classes

Class A Example

• Network: 10
• Hosts: 0.0.1

Note that the first octet (10) is between 1-126.

Class B Example

• Network: 172.16
• Hosts: 5.10

Note that the first octet (172) is between 128-191.

Class C Example

• Network: 192.168.1
• Host: .10

Note that the first octet (192) is between 192-223.

Limitations of Classful Addressing

Classful networking is simple but inefficient:

• Wasted IP space
  • A Class A network supports ~16 million hosts (far too large for most cases)
  • A Class B network supports ~65 thousand hosts (still to large for most cases)
  • A Class C network supports only 254 hosts (often too small)
• No flexibility
  • You couldn’t choose a subnet size between classes

4. CIDR (Classless Inter-Domain Routing)

Classful addressing only allows the network and host portions of an IP address to be defined in fixed 8-bit increments (i.e., by octet). This lack of flexibility often leads to inefficient use of IP address space.

To overcome these limitations, CIDR (Classless Inter-Domain Routing) was introduced.

CIDR removes the restriction of fixed class boundaries and allows networks to be defined using any number of bits for the network portion. Instead of being limited to 8, 16, or 24 bits, you can specify a network prefix of any length between 1 and 32 bits.

This enables much more efficient allocation of IP addresses, as subnet sizes can be tailored to match the exact requirements of a network.

CIDR Notation

CIDR uses a suffix like:

192.168.1.10/24

The /24 means:

• The first 24 bits are the network portion
• Equivalent subnet mask: 255.255.255.0

Common CIDR Examples

Example: Flexible Subnetting

Instead of being forced into classful subnet like Class C:

192.168.1.10/24  → 254 hosts

You can create more versatile subnets:

192.168.1.10/28 → 14 hosts

192.168.1.10/20 → 4096 hosts

This is especially useful for:

5. Mask Compatibilty

In classful addressing, subnet masks were tied directly to the IP address class. For example:

• Class A → /8 (255.0.0.0)
• Class B → /16 (255.255.0.0)
• Class C → /24 (255.255.255.0)

CIDR

With CIDR, this restriction no longer applies.

Any IP address can use any subnet mask, regardless of its original class.

For example:

• 192.168.1.1/24 → traditional Class C-sized network
• 192.168.1.1/28 → smaller subnet carved from the same range
• 10.0.0.1/24 → Class A address using a Class C-sized subnet
• 192.168.1.1/8→ Class C address using a Class A-sized subnet
• 172.16.0.1/20 → Class B address using a custom subnet size