IPv4 subnet masks are used to divide an IP address into network and host portions, facilitating the creation of subnetworks within a larger network. The subnet mask consists of a series of contiguous '1' bits followed by '0' bits, with the number of '1' bits indicating the size of the network portion.

Here's why we use subnet masks:

1. Network Segmentation: Subnet masks enable the segmentation of a larger IP address space into smaller, manageable subnetworks. This segmentation helps in organizing and optimizing network resources.

2. Efficient IP Address Allocation: By using subnet masks, administrators can allocate IP addresses more efficiently, assigning subsets of addresses to specific subnetworks based on their size and requirements.

3. Improved Security: Subnetting enhances network security by isolating different sections of a network. Communication between subnets may be controlled, reducing the attack surface and containing potential security breaches.

4. Broadcast Domain Control: Subnetting limits the scope of broadcast domains. Broadcast traffic is contained within a subnet, preventing it from unnecessarily traversing the entire network and causing congestion.

5. Optimized Routing: Subnetting allows for more efficient routing within a network. Routers can make decisions based on the subnet information, directing traffic more intelligently.

Overall, subnet masks play a crucial role in IP address management, network organization, and resource optimization within complex network infrastructures.