Subnet & IP Planning Calculator

CIDR, VLSM & VLAN Design: A Practical Reference for Network Engineers

The calculator above handles the math. This reference covers the reasoning behind the numbers — covering CIDR notation, subnet sizing rules, VLSM allocation strategy, and VLAN segmentation principles for enterprise deployments.

Understanding CIDR Notation and Subnet Masks

Classless Inter-Domain Routing (CIDR) replaced the old Class A / B / C system because fixed class boundaries wasted enormous amounts of address space. A Class B allocation handed out 65,534 host addresses regardless of whether the organization needed 50 or 50,000. CIDR fixes this with a trailing slash notation that defines exactly how many bits belong to the network identifier.

In a /n prefix, the first n bits identify the network and the remaining 32 - n bits address individual hosts within it. The usable host count follows the formula 2^(32-n) - 2, where the two subtracted addresses are the network identifier (all host bits zero) and the broadcast address (all host bits one), both of which are reserved by protocol and cannot be assigned to endpoints.

The CIDR Lookup tab above calculates all values for any prefix you enter.

RFC 1918 Private Address Space — Choosing the Right Range

RFC 1918 defines three private IPv4 ranges that are non-routable on the public internet. Routers at the internet boundary drop traffic sourced from or destined to these addresses, which is why NAT exists at every enterprise edge. The three ranges are not interchangeable — each has practical characteristics that make it more or less appropriate for a given deployment context.

VLSM: Carving Subnets the Right Way

Variable Length Subnet Masking (VLSM) allows different subnets within the same parent block to use different prefix lengths. This is the correct approach for any network with segments of varying sizes — assigning a /24 to every VLAN regardless of actual host count wastes address space and produces unnecessarily large broadcast domains.

The VLSM Planner tab implements the standard allocation algorithm: subnets are sorted largest-first, then each is assigned the smallest prefix that satisfies its host requirement, aligned to the next valid boundary within the parent block. Allocating largest-first minimizes address waste from alignment padding. The remaining space after all allocations is your growth buffer — plan to keep it above 30%.

Enter any parent block and required host counts in the VLSM Planner tab to get assigned CIDRs, usable ranges, and a space utilization summary automatically.

VLAN Segmentation: Why Traffic Separation Matters

A VLAN (Virtual LAN) is a logical Layer 2 broadcast domain defined by an 802.1Q tag. Devices on the same VLAN communicate directly at Layer 2 without passing through a router. Traffic between VLANs requires inter-VLAN routing — either a Layer 3 switch with SVIs (Switched Virtual Interfaces) or a dedicated router interface — which is where firewall policy and QoS enforcement happen.

Proper VLAN segmentation is not just an organizational convention. It is the enforcement boundary for QoS policies, security zones, and compliance controls. Without it, all traffic competes on the same broadcast domain and there is no clean point at which to apply differentiated treatment.

Load this scheme directly into the VLAN Planner tab using the Reset to Default Layout Template button.

Once your VLAN table is built, the Enterprise VLAN Config Pack has production-ready configuration templates for Cisco IOS and Aruba CX — VLAN creation, access ports, trunk ports, SVIs, and DHCP scopes. Swap in your subnets from the planner above and deploy.