Subnet Splitter & Merger

Subnet division (splitting) takes a larger network block and breaks it into smaller, equal-sized subnets. This is one of the most common tasks in network planning — whether you're allocating address space in a data centre, setting up VLANs in an office, or designing cloud VPC architectures.

The process works by borrowing bits from the host portion of an IP address and using them as subnet bits. Each additional bit you borrow doubles the number of subnets while halving the number of hosts per subnet. For example, splitting a /24 (256 addresses) into /25s gives you two subnets of 128 addresses each. Splitting further into /26s gives four subnets of 64 addresses.

At NetOz, we use subnet splitting to allocate address space across our Adelaide hosting infrastructure. A /24 block like 151.158.22.0/24 might be split into /28 subnets, giving each customer rack its own 14-host network segment with proper isolation.

When splitting subnets, remember that each subnet loses two addresses — the network address (first) and the broadcast address (last). A /28 has 16 total addresses but only 14 usable hosts. For very small subnets like /30 (used for point-to-point links), you get just two usable addresses, and /31 subnets (RFC 3021) can use both addresses by eliminating the broadcast.

Variable Length Subnet Masking (VLSM) is a technique that allows you to use different subnet sizes within the same network, rather than dividing everything into equal-sized blocks. This dramatically reduces IP address waste by matching the subnet size to the actual number of hosts needed.

Before VLSM, classful networking required all subnets within a network to use the same mask. If you had a department with 5 hosts and another with 200, both would get the same size subnet — wasting hundreds of addresses for the smaller department. VLSM, defined in RFC 1009 and widely adopted with CIDR (RFC 4632), solved this by allowing subnets of varying sizes.

The key to VLSM planning is to allocate the largest subnets first, then fill in smaller ones from the remaining space. Always start from the beginning of the address block and work forwards, ensuring subnets don't overlap. This tool's overlap checker helps you verify that your VLSM allocations are conflict-free.

VLSM requires routing protocols that carry subnet mask information — OSPF, EIGRP, IS-IS, and BGP all support VLSM. Older protocols like RIPv1 and IGRP do not, but these are rarely used in modern networks.

Network segmentation divides a larger network into isolated segments, improving security, performance, and manageability. By splitting subnets, you create boundaries that control traffic flow between segments and limit the blast radius of security incidents.

Effective segmentation relies on firewall rules or access control lists between subnets. Simply placing devices on different subnets doesn't provide security unless you also control routing and filtering between them. Combine subnetting with VLANs at layer 2 for complete isolation.