Subnet Planning & Calculators · draws the plan
Subnet planning and calculators for getting the plan right first
Most address conflicts are planned long before they happen. A branch office gets a /24 because that is what the last one got; two years later the voice VLAN needs room the plan never left, and someone squeezes a range into the gap between the DHCP pool and the printers. Subnet planning is where the record starts, and a mistake at this stage is copied into every reservation that follows.
The tools in this category help at two levels. A subnet calculator answers the arithmetic: network and broadcast addresses, usable host counts, how a /22 splits into /24s or /26s, what mask a given host count needs, and the IPv6 equivalents where a /64 per VLAN is the norm. IPAM products with prefix hierarchies go further — they hold the plan itself, show which blocks are still free inside a parent, and warn when a new subnet would overlap an existing one.
We rank these by how well the plan survives contact with the record: whether the planned subnets can be created where the addresses will later be tracked, whether overlap is caught, and whether free space is visible at a glance. A calculator alone scores lower on that yardstick, but it is often the fastest way to check a design on a Tuesday afternoon. See the methodology page for how each tool is assessed.
6 subnet planning tools side by side
Ordered by how well each tool keeps the subnet plan and the address record in one place, with a standalone calculator placed where its speed for quick arithmetic earns it. Tool names lead to our register entry for that product; the vendor link leaves for the publisher’s own site.
| Tool | Licence | Platforms | What it records well | Suits |
|---|---|---|---|---|
| NetBoxNetBox Labs | Apache 2.0 + paid | Self-hosted (Python/Django + PostgreSQL + Redis on Linux) or NetBox Cloud | Prefix hierarchy with VRFs, VLANs and utilisation, tied to a DCIM model and a full REST and GraphQL API | Teams that treat the network record as a source of truth for automation |
| phpIPAMphpIPAM project | GPLv3 | Self-hosted web app (PHP on Apache or nginx + MySQL/MariaDB) on Linux; Docker | Subnet tree with scheduled ping/scan agents that flag live hosts missing from the record | Small teams replacing an IP spreadsheet without buying a licence |
| LizardSystems LanCalculatorLizardSystems | Free personal · paid business | Windows 10/11, Windows Server 2012 R2–2019 | Splits IPv4 or IPv6 networks by prefix, subnet count or hosts per subnet, and exports the subnet list | Working out masks and ranges on paper before they go into the IPAM |
| SolarWinds IP Address ManagerSolarWinds | Commercial · trial | Windows Server 2012 R2–2025 with SQL Server; web console | Subnet scans plus Microsoft, ISC, Kea, Cisco and Infoblox DHCP/DNS integration, with conflict and scope alerts | Windows-centric shops with Microsoft DHCP/DNS that want alerts, not just a list |
| GestioIPGestioIP | GPLv3 | Self-hosted web app (Perl + Apache/mod_perl + MySQL/MariaDB) on Linux; Docker Compose | Discovery that fills the register from routing tables, SNMP, DNS and ping, plus DHCP lease import | Admins who want an open-source IPAM that populates itself from the network |
| ManageEngine OpUtilsManageEngine (Zoho Corp.) | Free edition + paid | Windows or Linux server; web console | IP address manager paired with a switch port mapper, so a live address leads to a physical port | Admins who need to trace an address to a switch port as well as record it |
TechScan365 compiled this table independently: we are none of the vendors listed, and no row was paid for. Licence and platform details were matched to each publisher’s website on the date in the byline.
How to choose
- Plan from the supernet down
Reserve a summarisable block per site — say a /20 — and carve VLAN subnets from it, rather than picking /24s one at a time. Summarised routes are easier to filter and firewall, and a hierarchy is what lets an IPAM show you free space inside the parent.
- Size for growth, not today’s headcount
Count devices per VLAN, then plan for at least double. A /26 gives 62 usable hosts, a /25 gives 126, a /24 gives 254. Wireless and guest networks churn through leases quickly, so give them more room and shorter lease times.
- Write the conventions down with the plan
Decide where gateways live (the first usable address is the common choice), which ranges are static, which are DHCP, and which are reserved for infrastructure. Put that convention in the IPAM or planning document, not in someone’s head.
- Check for overlap across sites and VPNs
192.168.1.0/24 is on every consumer router in the world, including the ones your remote staff sit behind. Avoid the common home ranges for corporate subnets, and let the planning tool flag any prefix that collides with an existing one before a site-to-site tunnel exposes it.
- Keep IPv6 in the same plan
Even if IPv6 is not deployed yet, allocate a /48 or /56 per site and a /64 per VLAN alongside the IPv4 plan. A tool that holds both families in one hierarchy saves a second migration later.
Where a calculator is enough
If you are designing one branch office and will type the result into an existing IPAM or a well-kept spreadsheet, a desktop calculator such as LanCalculator does the arithmetic quickly and without a server. LanCalculator is Windows-only and free for personal use, with a paid business licence. When several people plan subnets across many sites, move the planning into the IPAM itself, so that free-space views and overlap checks work from the same data your discovery scans reconcile against.
Vendor pages: NetBox netboxlabs.com · phpIPAM phpipam.net · LizardSystems LanCalculator lizardsystems.com · SolarWinds IP Address Manager solarwinds.com · GestioIP gestioip.net · ManageEngine OpUtils manageengine.com
Questions we get about subnet planning tools
What subnet size should a small branch office use?
A common pattern is a /22 or /23 per branch, split into /24s or smaller for users, voice, printers, management and guest Wi-Fi. That leaves room for growth and keeps the site summarisable to one route. Our guide on planning subnets and VLANs for a new site works through an example.
How many usable hosts are in a /24, /25 or /26?
Subtract the network and broadcast addresses from the block size: a /24 has 254 usable hosts, a /25 has 126, a /26 has 62, a /27 has 30. Point-to-point links often use /31 (two usable addresses under RFC 3021) to avoid wasting space.
Should every VLAN get a /24?
It is a readable convention, because the third octet then identifies the VLAN, and many teams keep it for that reason. It wastes space on small segments like management or out-of-band networks, where a /27 or /28 is plenty. Choose deliberately and write the rule down.
Can I plan IPv6 with the same tools?
Most tools here handle IPv6 prefixes. The planning rules differ: every LAN segment gets a /64, sites get a /48 or /56, and host counts stop mattering. What still matters is a clean hierarchy that matches your sites and VLANs.
Keep going
Other categories
- IPAM Software — holds the record
- Address Discovery Scanners — reads the wire
Disclosure: every vendor link here points directly at the publisher’s official site, and we are paid nothing when you use one. Details are in our affiliate disclosure.