Subnetting Interview Questions and Answers
Subnetting is the number-one networking interview topic — and the one candidates most often fumble under pressure. These are the questions interviewers actually ask, with the reasoning that gets you the right answer fast. Practise the maths live with our subnet calculator.
How to prepare
The secret to subnetting interviews is speed without a calculator. Master the block-size method (block = 256 − mask octet) and the powers of two until subnetting any address takes under 30 seconds. Interviewers watch how you work, not just the answer — narrate your steps. Drill on the practice tests.
Frequently asked questions
How many usable hosts are in a /26?
62. A /26 has 6 host bits: 2^6 − 2 = 62 (minus network and broadcast addresses).
What is the subnet mask for a /27?
255.255.255.224 — three subnet bits borrowed in the last octet (128+64+32).
What is the network address of 192.168.1.100/26?
192.168.1.64. Block size is 64, so networks fall at .0, .64, .128, .192 — and .100 sits in the .64 block.
What is the broadcast address of 10.0.0.0/30?
10.0.0.3. A /30 has a block size of 4 (.0–.3), so broadcast is the last address, .3.
Why do we subtract 2 when counting hosts?
The network address and the broadcast address can't be assigned to devices, so usable hosts = 2^(host bits) − 2.
What is VLSM?
Variable Length Subnet Masking — using different subnet sizes within one network so each subnet is exactly as large as needed, avoiding wasted addresses.
How many subnets does borrowing 3 bits create?
2^3 = 8 subnets. Borrowed bits determine subnet count (2^n); remaining host bits determine hosts per subnet.
What is a /30 used for?
Point-to-point links between routers — it provides exactly 2 usable hosts, perfect for a two-router connection.
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