Subnet Count Calculator
Calculate how many subnets you get by borrowing bits from the host portion of an address.
Inputs
Number of host bits converted to subnet bits
Subnets Created
8
New Prefix
/27
Hosts per Subnet
30
Step by step
Values used
Original Prefix = 24 /; Bits Borrowed = 3
Subnets from borrowed bits
subnets = 2^borrowed_bits
Subnets Created
= 8
New Prefix
= /27
Hosts per Subnet
= 30
How it works
Subnetting works by borrowing bits from the host portion of an address and reassigning them to the network portion. Borrowing n bits multiplies the number of subnets by 2^n while dividing the hosts available per subnet accordingly. The new prefix length is simply the original prefix plus the number of borrowed bits.
Formula
Subnets from borrowed bits
subnets = 2^borrowed_bits
- n
- Number of bits borrowed from the host portion
Frequently Asked Questions
What does 'borrowing bits' mean?
It means taking bits that were previously part of the host identifier and using them instead to identify individual subnets, which increases the prefix length by that many bits.
How many subnets does borrowing 3 bits create?
2^3 = 8 subnets. Each additional borrowed bit doubles the subnet count while halving the hosts available in each subnet.
Is there a tradeoff when borrowing more bits?
Yes — every bit borrowed for subnets is a bit no longer available for hosts, so you gain more, smaller subnets at the cost of fewer usable addresses per subnet.