Subnet Split Count Calculator
Split an IPv4 network into smaller subnets: get the number of subnets created, addresses per subnet and usable hosts when moving from one prefix to another.
How it works
Subnets = 2^(new − original) · Addresses per subnet = 2^(32 − new) · Usable hosts = addresses − 2
Borrowing bits from the host portion of an address subdivides one network into many. Each extra bit in the prefix doubles the subnet count, so going from /16 to /24 borrows 8 bits and yields 2^8 = 256 subnets. Every subnet still reserves two addresses — the network (all-zero host) and broadcast (all-one host) — so usable hosts are the block size minus two, except /31 point-to-point links and /32 host routes which follow special rules.
Worked example
From a /16 to /24 you borrow 24 − 16 = 8 bits, creating 2^8 = 256 subnets. Each /24 holds 2^(32−24) = 256 addresses, of which 254 are usable hosts — so 256 × 254 = 65,024 usable hosts across the whole block.
Frequently asked questions
Why subtract 2 from the address count?
Each subnet reserves the first address as the network identifier and the last as the broadcast address, so those two cannot be assigned to hosts. A /24 with 256 addresses therefore offers 254 usable host addresses.
What about /31 and /32?
A /31 (RFC 3021) is used for point-to-point links and provides 2 usable addresses with no separate broadcast, while a /32 is a single host route with 1 address. This calculator applies those special cases automatically.
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This calculator is for educational purposes. Double-check important results before acting on them.