thinQmaster Virtual testing partner Adaptive practice exams that find your weak spots and drill them. Every answer explained. From $69
Enroll

thinQmaster

Find your exam

Five questions free, before you decide.

Step 1 · choose a vendor

In one of our camps? Your practice exams come with the tuition — they are on your study page already.

Networking Fundamentals · Foundational

Number of Subnets Formula

The calculation 2^s, where s is the number of borrowed subnet bits, giving how many subnets are created.

Definition

Borrowing s bits from the host portion yields 2^s subnets. Older classful teaching subtracted 2 to exclude subnet zero and the all-ones subnet, but modern equipment uses both, so 2^s is the current answer. Combine with the hosts formula to choose a mask that meets both requirements.

Example

Borrowing 3 bits from a /24 creates 8 subnets of /27.

Exam tip

Modern exams use 2^s, not 2^s - 2, for subnets.

Related terms