IP Subnetting & CIDR VLSM Calculator

Historical IP routing relied on classful addressing (Class A, B, and C networks), which had fixed mask lengths (/8, /16, and /24). This structure was highly inefficient and wasted millions of IP addresses. In 1993, Classless Inter-Domain Routing (CIDR) was introduced, allowing flexible subnet masks of any bit length.

CIDR uses slash notation (e.g., /24) to specify the number of leading network bits in the mask. A /24 subnet has 24 network bits (ones in binary) and 8 host bits (zeros), which translates to a subnet mask of 255.255.255.0. The number of usable hosts in any CIDR block is calculated as: Usable Hosts = 2^(32 - CIDR) - 2, where 2 is subtracted for the network and broadcast addresses.

Variable Length Subnet Masking (VLSM) allows network administrators to allocate subnets of varying sizes from a single base IP block, matching the exact host requirements of each sub-network. This prevents address waste by matching masks to host capacities.

The VLSM allocation algorithm requires sorting the host requirements from largest to smallest. The base IP is assigned to the largest subnet block, using a mask size that accommodates the hosts. The next subnet is then allocated immediately following the end of the previous subnet, preventing IP address overlapping: Next Network = Current Network + Block Size.

A wildcard mask is the bitwise inverse of a subnet mask, calculated as: Wildcard = 255.255.255.255 - Subnet Mask. For example, a subnet mask of 255.255.255.240 has a wildcard mask of 0.0.0.15. Wildcard masks are used in Cisco Access Control Lists (ACLs) to specify which bits of an IP address must match.

To calculate corporate cloud configurations and routing, you can combine this calculator with our IP subnet calculator to analyze single CIDR blocks, or estimate data capacity and bandwidth needs using the network throughput calculator to size infrastructure WAN connections.

In Single Subnet mode, enter a base IP address and CIDR prefix to see the network address, subnet mask, wildcard mask, broadcast address, and usable host range. In VLSM Planner mode, enter the host count needed for each sub-network and add or remove rows to model your full address plan.

VLSM mode automatically sorts and allocates subnets from the base block without overlap, rounding each request up to the nearest power-of-two block size.

The default single-subnet view for 192.168.1.0/24 shows a network address of 192.168.1.0, subnet mask 255.255.255.0, broadcast 192.168.1.255, and 254 usable hosts (192.168.1.1–192.168.1.254).

Splitting that same /24 block via VLSM for three departments needing 50, 20, and 10 hosts allocates a /26 (62 usable hosts, 192.168.1.0–192.168.1.63), a /27 (30 usable hosts, 192.168.1.64–192.168.1.95), and a /28 (14 usable hosts, 192.168.1.96–192.168.1.111), each block sized to the nearest power of two above the request, with no address overlap.

For other structured-data validation tasks common in networking documentation, see the JSON Formatter & Validator or the Regular Expression Tester. When a block only needs to be split into equal parts, or you want the network and broadcast addresses of a CIDR range, the IP CIDR range calculator does that directly.