Subnetly

Split a CIDR block into subnets, see the bit-level math behind every mask, and test yourself with a quick practice quiz.

Network input

Efficient allocation: use VLSM when subnet sizes differ. Enter the host counts needed by each department and the calculator will allocate the smallest fitting blocks, leaving the remaining addresses available for future networks.

Bit-level breakdown

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Network bits Host bits

Drag the slider to see how the network/host boundary shifts — this is exactly what a router looks at when it applies a subnet mask.

Private network classes — efficient allocation examples

For practical private IPv4 planning, this tool uses the familiar private blocks 10.0.0.0/8 (A-style), 172.16.0.0/12 (B-style), and 192.168.0.0/16 (C-style). Choose the block that fits the network size, then subnet it with the smallest blocks that satisfy each host requirement. Use VLSM when different departments need different numbers of hosts so unused addresses are minimized.

Class A
Private block: 10.0.0.0/8
Large private address space. Start from 10.0.0.0 and choose a longer prefix such as /12, /16, /20, or /24 according to the number of hosts needed.
10.0.0.0/12
Class B
Private block: 172.16.0.0/12
Medium-sized private address space. Start from 172.16.0.0 and extend the prefix to allocate only the address space each subnet needs.
172.16.0.0/20
Class C
Private block: 192.168.0.0/16
Smaller private networks. Start from 192.168.0.0 and use /24, /25, /26, /27, etc. to match the actual host requirement.
192.168.0.0/26
Class D
224.0.0.0 – 239.255.255.255
Reserved for multicast groups, not individual hosts — there's no default mask or subnetting here.
Class E
240.0.0.0 – 255.255.255.255
Reserved for experimental use. Not allocated for general host addressing.

Test what you've learned

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