User Guide
Everything you need to plan and document your VLAN & subnet structure — IPv4 and IPv6.
Quick Start
- 1
Choose IP version
Select IPv4 or IPv6 using the toggle next to the Parent Network field. Each plan remembers its IP version. Switching resets the parent network to a sensible default.
- 2
Set your parent network
Type the top-level network block your subnets live in (e.g. 10.0.0.0/8 or 2001:db8::/32) and click Set Network. The tool shows the full address range and host count.
- 3
Add subnets
Click "Add subnet" or press Ctrl+Enter to add a new row. Fill in Network (CIDR), VLAN ID, Name, Purpose, and Gateway. The gateway is auto-filled with the first usable address.
- 4
Check the status column
Each row shows a live status icon. Green = valid, red = overlapping with another subnet, yellow = outside the parent network, grey = invalid CIDR.
- 5
Export your plan
Click CSV to download a spreadsheet-compatible file, or PDF (Pro) for a formatted document ready to share or print.
IPv4 CIDR Quick Reference
CIDR notation expresses a network as address/prefix where the prefix is the number of network bits.
| CIDR | Subnet Mask | Usable Hosts | Typical Use |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,214 | Large enterprise / ISP |
| /16 | 255.255.0.0 | 65,534 | Corporate / campus |
| /20 | 255.255.240.0 | 4,094 | Large VLAN |
| /22 | 255.255.252.0 | 1,022 | Medium VLAN |
| /24 | 255.255.255.0 | 254 | Standard VLAN |
| /25 | 255.255.255.128 | 126 | Split /24 |
| /26 | 255.255.255.192 | 62 | Small VLAN |
| /28 | 255.255.255.240 | 14 | Management / point-to-point |
| /30 | 255.255.255.252 | 2 | WAN link / router interconnect |
| /31 | 255.255.255.254 | 2 | Point-to-point (RFC 3021) |
| /32 | 255.255.255.255 | 1 | Host route / loopback |
IPv6 CIDR Quick Reference
IPv6 uses 128-bit addresses in hexadecimal groups separated by colons. A :: shortens consecutive all-zero groups. The prefix works the same as IPv4 — it defines how many bits are the network part.
| Prefix | Addresses | Typical Use |
|---|---|---|
| /32 | 2^96 (~79 octillion) | ISP allocation — delegated to customers |
| /48 | 2^80 (~1.2 sextillion) | Enterprise / site allocation |
| /56 | 2^72 | Residential ISP prefix |
| /64 | 2^64 (~18 quintillion) | Standard subnet — one per VLAN |
| /80 | 2^48 | Large segment within a /64 |
| /96 | 2^32 | IPv4-mapped / special use |
| /112 | 2^16 (65,536) | Management / point-to-point |
| /126 | 4 | Router interconnect (like /30) |
| /127 | 2 | Point-to-point (RFC 6164) |
| /128 | 1 | Host route / loopback |
Common IPv6 address ranges
2000::/3 — Global unicast (public)fc00::/7 — Unique local (ULA, like RFC1918)fe80::/10 — Link-local (auto-assigned)ff00::/8 — Multicast2001:db8::/32 — Documentation / examples::1/128 — LoopbackFeatures
IPv4 & IPv6 support
Switch between IPv4 and IPv6 per plan using the toggle next to the parent network. Full CIDR parsing, overlap detection, and gateway validation for both.
Overlap detection
Rows turn red when two subnets overlap. Fixed in real time as you type.
Parent validation
Yellow warning when a subnet falls outside the defined parent network block.
Auto-fill gateway
When you type a CIDR, the gateway field is populated with the first usable IP. Works for both IPv4 (.1) and IPv6 (first host in subnet).
Next free subnet
Toolbar button auto-calculates the next available block at a chosen prefix length — no manual math. Supports IPv4 (/24–/30) and IPv6 (/48–/112).
Address Space bar
Visual bar showing how your subnets occupy the parent block (IPv4 only). Hover a segment to see its details.
Stats dashboard
Live counts of defined subnets, allocated IPs, free IPs, and utilization percentage (IPv4 only).
Drag & drop rows
Reorder subnets by dragging the grip handle on the left of each row.
Multiple plans
Create separate plans for different sites, customers, or environments. Free tier: 2 plans. Pro: unlimited.
Auto-save
Your plans are automatically saved in the browser and survive page refreshes.
CSV export
Exports all rows with calculated fields. Import into Excel or Google Sheets.
PDF export
Formatted landscape table ready to attach to documentation. Pro feature.
Keyboard Shortcuts
Tab / EnterMove to the next cell in the rowShift+Tab / Shift+EnterMove to the previous cellCtrl+Enter (⌘+Enter on Mac)Add a new empty row at the bottomEnter on last cellAdds a new row and focuses its first fieldTips & Best Practices
IPv4
- ›Always define a parent network first — it enables the visual bar, stats, and outside-parent warnings.
- ›Use /24 (255.255.255.0) for standard user VLANs — 254 hosts is enough for most segments.
- ›Assign management networks a /28 or /29 to limit the blast radius if something goes wrong.
- ›Use /30 for WAN links and router-to-router interconnects — only 2 hosts needed.
- ›Keep VLAN IDs consistent with your switch configuration. VLAN 1 is usually the native/default VLAN — avoid using it for production traffic.
- ›Export to PDF before making changes — it acts as a snapshot of your current plan.
IPv6
- ›Use a /48 as your site parent (common ISP delegation) and assign a /64 to each VLAN — one /64 per segment is the standard.
- ›Every /64 has 2^64 addresses — there is no concept of "running out" within a segment. No need to use /126 or /127 for LANs as you would in IPv4.
- ›Use /127 for router interconnects (RFC 6164 equivalent of /30) and /128 for loopbacks.
- ›Unique Local Addresses (fc00::/7) are the IPv6 equivalent of RFC1918 private space — use them for internal infrastructure.
- ›Never use link-local (fe80::/10) as a routed subnet — it is automatically assigned per interface and not globally routable.
- ›The gateway in an IPv6 subnet is typically the first address (.::1) — the tool auto-fills this when you enter a CIDR.
FAQ
Why does my CIDR show a different network address than what I typed?
The tool normalises your input to the proper network address. For example, 192.168.1.5/24 becomes 192.168.1.0/24 — the host bits are zeroed out. Same for IPv6: 2001:db8::5/48 becomes 2001:db8::/48. This is correct CIDR behaviour.
How do I switch between IPv4 and IPv6?
Use the IPv4 / IPv6 toggle next to the Parent Network field. Switching resets the parent network to a default example for the selected version (192.168.0.0/16 for IPv4, 2001:db8::/32 for IPv6). Each plan remembers its IP version.
Why is the Address Space bar not showing in IPv6 mode?
IPv6 subnets contain up to 2^128 addresses — numbers too large for precise pixel-level rendering. The bar is only available in IPv4 mode. Overlap detection, parent validation, and gateway validation work in both modes.
My plan disappeared after closing the browser. What happened?
Plans are saved in browser localStorage. If you cleared your browser data, the plan was deleted. Use the PDF or CSV export to keep a permanent copy.
Can I have more than one plan?
Yes — use the plan selector in the header to create, rename, and switch between multiple plans. Each plan has its own parent network, IP version, and subnet table. Free tier allows 2 plans; Pro unlocks unlimited.
What does the gateway auto-fill set?
For IPv4 it sets the first usable host in the subnet (x.x.x.1 for a /24). For IPv6 it sets the first host address in the subnet (network address + 1). You can overwrite either if your gateway is on a different address.
What is the "Next free subnet" button?
It automatically finds the next available block at a chosen prefix length within the parent network, skipping over any already-defined subnets. Choose the prefix from the dropdown and the tool adds the new row with CIDR and gateway pre-filled.
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