What is an IPv6 visual subnet calculator?
An IPv6 address plan is mostly a question of cutting a big prefix into smaller ones. A visual subnet calculator shows that plan as a map. You start with the prefix you were given, cut it into smaller pieces and see at once where each piece begins and ends and how many /64 networks it holds.
This one has two views of the same plan. The subnet map is a table with a Hierarchy (Split/Join) column: a split button on every subnet and join buttons that group the subnets that belong together. The hierarchy is a tree that shows each subnet under the network it was cut from.
How to use the IPv6 Visual Subnet Calculator
Type the Network Address and set the Prefix Length with the slider. You can also paste an address with its prefix, like 2001:db8:acad::/48, into the first box. If the address is inside a network but is not its first address, the calculator uses the network address and tells you.
The result shows straight away and follows every change you make. At first the map holds one subnet: your whole network. Each subnet shows its first and last address, how many addresses it holds and how many /64 networks it contains.
A teal button splits a subnet in two. It sits in the Hierarchy (Split/Join) column on the right of the table and shows the subnet's own prefix, like /48. Click it to split that subnet into two equal halves, here two /49 subnets. Each subnet is split on its own, so you can end up with subnets of different sizes. To cut everything to one size in one go, use Divide all to.
A blue button joins subnets back together. It sits next to the group of subnets it covers and shows the size you get when you click it. The buttons closest to the subnets join two halves, and the buttons further right join bigger groups. Join All puts the whole network back together.
Tick or untick boxes in Show Columns to choose what the map shows. Tick Note to type a label next to each subnet, such as "Building A" or "Servers".
Copy puts the subnet map and the hierarchy on your clipboard and Download saves them as a CSV file. Both include every subnet and the columns you ticked. Clear All puts the form back to its starting values.
How splitting and joining works
A subnet with prefix /n holds 2(128 − n) addresses. Splitting it adds one bit to the prefix, which gives two subnets of half the size:
Joining does the opposite. The two halves become the parent again, with the prefix /(n − 1) and the network address of the first half. Only two subnets that were cut from the same parent can be joined, which is why the join buttons always cover whole groups.
IPv6 has no broadcast address, so every address in a subnet can be used. That is why the map shows first and last address and the number of /64 networks instead of host counts.
Example: split a /48 into three subnets
Start with 2001:db8:acad::/48. Split it into two /49 subnets, then split the second /49 into two /50 subnets. The map ends with three subnets:
| Network Address | First Address | Last Address | Total Addresses | Exact Number of Addresses |
|---|---|---|---|---|
| 2001:db8:acad::/49 | 2001:db8:acad:: | 2001:db8:acad:7fff:ffff:ffff:ffff:ffff | 2^79 (≈ 6.04×10^23) | 604,462,909,807,314,587,353,088 |
| 2001:db8:acad:8000::/50 | 2001:db8:acad:8000:: | 2001:db8:acad:bfff:ffff:ffff:ffff:ffff | 2^78 (≈ 3.02×10^23) | 302,231,454,903,657,293,676,544 |
| 2001:db8:acad:c000::/50 | 2001:db8:acad:c000:: | 2001:db8:acad:ffff:ffff:ffff:ffff:ffff | 2^78 (≈ 3.02×10^23) | 302,231,454,903,657,293,676,544 |
| Level | Subnet | Type | Range of Addresses |
|---|---|---|---|
| 1 | 2001:db8:acad::/48 | Parent | 2001:db8:acad:: – 2001:db8:acad:ffff:ffff:ffff:ffff:ffff |
| 2 | 2001:db8:acad::/49 | Subnet | 2001:db8:acad:: – 2001:db8:acad:7fff:ffff:ffff:ffff:ffff |
| 2 | 2001:db8:acad:8000::/49 | Parent | 2001:db8:acad:8000:: – 2001:db8:acad:ffff:ffff:ffff:ffff:ffff |
| 3 | 2001:db8:acad:8000::/50 | Subnet | 2001:db8:acad:8000:: – 2001:db8:acad:bfff:ffff:ffff:ffff:ffff |
| 3 | 2001:db8:acad:c000::/50 | Subnet | 2001:db8:acad:c000:: – 2001:db8:acad:ffff:ffff:ffff:ffff:ffff |
The hierarchy shows that the first /49 was left whole, while the second /49 became a parent with two /50 subnets under it.
When it helps
Use it to turn a /48 or /56 from your provider into an address plan, to give each building, VLAN or customer its own block, to plan a cloud VPC range, and to check your answers when you study for CCNA. Because the plan is a map, you can try a layout, change your mind in one click and copy the result into your documentation.
To look at one IPv6 address or prefix in detail, use the IPv6 Calculator. For IPv4, use the Visual Subnet Calculator.