VLSM Subnet Calculator

📖 Tool Introduction

VLSM subnetting is one of the most common calculation problems network engineers face both in certification exams and on the job. Given a base network such as 192.168.1.0/24 and a list of host counts required by several subnets, how do you carve them up from largest to smallest so that every subnet is just big enough without wasting address space? That is exactly what Variable Length Subnet Masking is about. This tool moves the whole bit-level procedure into your browser: you type the base CIDR and one host requirement per line, and the tool automatically sorts the requirements descending, works out the required host bits for each subnet, then computes its network address, subnet mask, usable host range and broadcast address, and presents everything in one clean plan table. Every figure is derived with 32-bit unsigned integer shifts and masking operations, so the result is exact down to the last IP address rather than an approximation. This is the same calculation taught in CCNA-level networking courses and used by real network administrators every day when breaking a large block of addresses into department-sized subnets. It is easy to make a mistake by allocating the smallest subnet first, which fragments the address space and leaves no room for the larger requirements; the tool sorts everything for you and follows the largest-first rule automatically. It also warns you clearly when a single requirement is too big for the parent network, or when the remaining address space runs out halfway through the list, instead of silently producing a broken plan. You can copy the finished table as CSV and drop it straight into a design document, a change ticket, or an IPAM import, which turns what used to be a twenty-minute pencil exercise into a few seconds of work.

✨ Key Features

• Auto sorting: host requirements are sorted largest-first, following VLSM best practice
• Exact bit math: network and broadcast addresses come from shift and mask operations with zero rounding error
• Full plan table: each row shows prefix, mask, network, usable range and broadcast
• Capacity checks: friendly errors when a requirement exceeds the base network or space runs out
• Copy and export: the plan can be copied as text or saved as CSV for design documents
• Built-in example: one click loads the classic 192.168.1.0/24 exercise

📝 How to Use

1. Type the base network in CIDR form, for example 192.168.1.0/24
2. Enter one required usable host count per line in the requirements box
3. Click Subnet and the plan table is generated below
4. Read each subnet's network address, mask, usable range and broadcast
5. Click Copy plan or Download CSV to reuse the result in your network design doc

⚠️ Notes

• Usable hosts: the network and broadcast addresses of each subnet cannot be assigned to hosts; usable hosts equal 2^h minus 2
• Allocation order: VLSM requires the largest requirement first, otherwise address fragmentation and waste appear
• Capacity limits: if a requirement exceeds the base network size, the tool reports an error instead of producing a wrong answer
• CIDR format: the base network must be written as IP/prefix, for example 10.0.0.0/16; a bare IP is not accepted
• Fully offline: all arithmetic runs locally in the browser with no network request at all