Subnetting in IPv4 is done to:

1. Organize Networks: Subnetting allows for the organization of larger IP address spaces into smaller, more manageable subnetworks. Each subnet can represent a distinct segment of a network.

2. Efficient IP Address Allocation: It enables more efficient use of IP addresses by assigning subsets to specific subnetworks based on their size and requirements, preventing waste of address space.

3. Enhance Security: Subnetting enhances security by isolating different sections of a network. Communication between subnets can be controlled, reducing the potential for unauthorized access.

4. Optimize Network Performance: It helps optimize routing, control broadcast domains, and manage network traffic more effectively, contributing to improved overall network performance.

5. Facilitate Scalability: Subnetting provides a scalable approach to network design, making it easier to expand and adapt to changing network requirements without major restructuring.

Overall, subnetting is a fundamental practice for effective IP address management and network organization. It brings flexibility, security, and efficiency to network design and administration.