SUBNET PLANNING

VLSM Calculator | VLSM Subnet Planner

02/ 06

Plan VLSM subnets from host requirements and calculate CIDR blocks, usable hosts and remaining address space.

Calculator inputs

IPv4

Calculation results

ALLOCATED SUBNETS

4 subnets

16unallocated addresses

192.168.1.0/24 · 240 / 256 addresses allocated

Request	CIDR	Need / usable	Host range
#1	192.168.1.0/25	100 / 126	192.168.1.1 192.168.1.126
#2	192.168.1.128/26	50 / 62	192.168.1.129 192.168.1.190
#3	192.168.1.192/27	25 / 30	192.168.1.193 192.168.1.222
#4	192.168.1.224/28	10 / 14	192.168.1.225 192.168.1.238

Conventional LAN allocation reserves network and broadcast addresses. Minimum block size is /30.

HOW IT WORKS

Allocate the largest VLSM subnets first

VLSM assigns a different mask to each subnet. This planner adds two addresses for the network and broadcast, then selects the smallest power-of-two block that fits.

In a /24, requirements of 100, 50, 25 and 10 hosts receive /25, /26, /27 and /28 blocks. Larger blocks are allocated first; request numbers retain the original input order.

This planner uses conventional LAN allocation with a minimum /30. Use the IPv4 calculator for /31 point-to-point links. Include gateways, management addresses and growth in your host requirements.

RFC 1812 · IPv4 routers ↗

FAQ

Frequently asked questions

Are requests allocated in input order?

Requests are sorted by block size, largest first. Request numbers in the table show the original input order.

What happens if the parent network is too small?

The calculator reports insufficient space instead of presenting a partial plan. Increase the parent network or reduce the requirements.

Do one or two hosts still receive a /30?

Yes. The conventional LAN model reserves network and broadcast addresses, so it allocates at least four addresses.