Why Subnetting Is Needed and How to Perform IPv4 Subnet Division, VLSM, and CIDR Calculations

1. Why divide into subnets?

IPv4 addresses, if only divided by class (A, B, C), lead to significant waste or shortage. To address this, the host portion of an IP address can be further split, moving part of it into the network portion, enabling networks of various sizes.

2. IPv4 Subnet Division and Aggregation

To overcome IPv4 limitations and increase flexibility, two important technologies were created: VLSM (Variable Length Subnet Mask) and CIDR (Classless Inter‑Domain Routing). They transform the traditional classful network model into a more efficient classless one.

VLSM is used for subdividing a large network into multiple smaller subnets, while CIDR is used for aggregating subnets, mainly in routing, by summarizing several small network routes into a single larger route, thereby reducing the number of routing entries and improving router efficiency.

3. IP Subnet Calculation

Algorithm: Number of usable hosts = 2 Y − 2 (subtract 2 for network and broadcast addresses), where Y is the count of “0” bits in the subnet mask.

Example: 202.106.1.0/24 is a C‑class public address block. How many IP addresses does this block contain?

Network address: 202.106.1.0 + 1 = 202.106.1.1

What is the last usable IP? It is 202.106.1.128.

Algorithm: Next network address (202.106.1.128) − 2 = 202.106.1.126

Broadcast address: Next network address − 1 = 202.106.1.127

The largest practical subnet size is /30; beyond that there are no usable host addresses.