VLSM Subnet Calculator Online
Plan variable-length subnets by specifying host requirements for each segment. The calculator optimally divides your address space — all processing happens client-side.
IP / CIDR Toolkit
Subnet calculator, VLSM divider, IP range converter, and IP address classifier — all in one toolkit.
Common Subnets
▶CIDR Reference Table
| CIDR | Subnet Mask | Addresses | Usable Hosts |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
| /9 | 255.128.0.0 | 8,388,608 | 8,388,606 |
| /10 | 255.192.0.0 | 4,194,304 | 4,194,302 |
| /11 | 255.224.0.0 | 2,097,152 | 2,097,150 |
| /12 | 255.240.0.0 | 1,048,576 | 1,048,574 |
| /13 | 255.248.0.0 | 524,288 | 524,286 |
| /14 | 255.252.0.0 | 262,144 | 262,142 |
| /15 | 255.254.0.0 | 131,072 | 131,070 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /17 | 255.255.128.0 | 32,768 | 32,766 |
| /18 | 255.255.192.0 | 16,384 | 16,382 |
| /19 | 255.255.224.0 | 8,192 | 8,190 |
| /20 | 255.255.240.0 | 4,096 | 4,094 |
| /21 | 255.255.248.0 | 2,048 | 2,046 |
| /22 | 255.255.252.0 | 1,024 | 1,022 |
| /23 | 255.255.254.0 | 512 | 510 |
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
| /31 | 255.255.255.254 | 2 | 2 |
| /32 | 255.255.255.255 | 1 | 1 |
About Subnet Calculation
- CIDR notation combines an IP address with a prefix length (e.g.,
192.168.1.0/24). - The prefix length defines how many bits are the network portion — the rest are host bits.
- Network address = all host bits set to 0. Broadcast = all host bits set to 1.
- Usable hosts = total addresses minus network and broadcast (except /31 and /32 special cases).
- Private ranges (RFC 1918): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
- Everything runs in your browser — no data is sent over the network.
What is VLSM?
VLSM (Variable Length Subnet Masking) allows different subnets within the same network to use different prefix lengths. Unlike fixed-length subnetting where every subnet is the same size, VLSM lets you allocate a /28 (14 hosts) for a small office and a /22 (1,022 hosts) for a data center floor — all from the same address block. This minimizes IP address waste.
Common use cases
Network architects use VLSM when designing enterprise networks with segments of varying sizes — server rooms, employee floors, IoT subnets, and point-to-point WAN links. CCNA and CCNP candidates practice VLSM calculations extensively. Cloud engineers apply VLSM principles when subdividing VPC address space across multiple availability zones and workload types.
Frequently Asked Questions
How does VLSM differ from fixed-length subnetting?
Fixed-length subnetting divides a network into equal-sized subnets, wasting addresses when segments vary in size. VLSM assigns different prefix lengths to each subnet based on actual host requirements, making much more efficient use of the address space.
What routing protocols support VLSM?
Classless routing protocols — OSPF, EIGRP, BGP, IS-IS, and RIPv2 — all support VLSM by including the subnet mask in routing updates. Classful protocols like RIPv1 do not support VLSM.