What is Subnetting?

Subnetting (sub + network) is the practice of dividing one large IP network into smaller subnets. This lets you match address counts to real needs and avoid unused IPs.

Figure 1 – Subnetting a /24 into four /26 networks.

For example, if you have 192.168.1.0/24 (256 addresses), subnetting lets you split it into four /26 subnets, each with 64 addresses.

In this lesson, you’ll see why subnetting was created. First, we’ll look at how IPv4 addressing worked before subnetting and the problems it caused. Then, as you progress through the pathway, you’ll learn how to subnet step by step.

Let me take you back more than 40 years, to 1981, when IPv4 was released, and I’ll show you why subnetting was created.

Back then, addresses were classified into A, B, C, D, and E. Each class had a first-octet range, a default mask, and a typical host capacity per network.

Table 1 – IPv4 address classes

At that time, companies requested address blocks from IANA (Internet Assigned Numbers Authority), which managed the global pool and ensured each allocation was unique.

For example:

Figure 2 – IANA assigns Class A 15.0.0.0/8, Class B 172.50.0.0/16, Class C 198.51.100.0/24.

• A large company needing about 1,000,000 IPs could receive a Class A (≈16.7M usable).

• A company needing around 50,000 IPs could receive a Class B (≈65.5k usable).

• A small company could receive a Class C (254 usable).

This worked at the very beginning of the Internet. But usage grew much faster than expected leading to problems you’ll see in the next section.

Classful addressing isn’t optimized for real network needs. Fixed block sizes with IPv4 classes mean you reserve far more IP addresses than you actually use, leading to significant waste.

Example 1 — Class C (/24) on a point-to-point link

Imagine you’re the network engineer tasked with assigning a network between two routers for a point-to-point link.
Under the classful model, you might assign a Class C /24, for example: 198.51.100.0/24.