Does this calculator configure prefix delegation on my router?

No. Prefix delegation is handled by your router and the service that assigns its prefix, often your internet provider. This calculator only calculates boundaries and sizes from the values you enter.

Should I use any calculated prefix length for SLAAC?

No. Stateless address autoconfiguration (SLAAC) normally requires a /64 prefix on common IPv6 links. A mathematically valid subdivision may not work with that address configuration method.

Can I use the same subnet prefix on two separate links?

Normally, each subnet prefix belongs to one link. Using the same prefix on separate links can cause routing and neighbor discovery problems, and this calculator does not check your network topology.

How can I check whether two calculated prefixes overlap?

Compare their first and last addresses, or determine whether one prefix contains the other. The form calculates one parent block at a time and does not automatically compare two independent plans.

Will my subnet plan still work if my internet provider changes my prefix?

If the new allocation has the same prefix length, you can reuse the same subnet sizes, but you must recalculate the actual addresses. If the allocation is smaller, check that all planned subnets still fit.

Can I calculate unique local IPv6 prefixes here?

Yes. The calculator can work with valid unique local addresses and their prefixes, but it does not generate a unique local allocation or check for conflicts with another network.

Why can a spreadsheet alter a copied address count?

A spreadsheet may round a very large integer when it interprets the cell as a number. To preserve every digit, format the destination cell as text before pasting the count.

Does the subnet list identify the best prefix for each department?

No. The list gives equally sized numerical blocks. Assigning them to departments or sites requires your own allocation plan and capacity decisions.

Will changing from compressed to expanded input change the network?

No, provided both inputs represent the same IPv6 value and use the same prefix length. Text formatting does not move the network boundary.

Can a prefix length end partway through a hexadecimal group?

Yes. A prefix counts bits rather than written groups, so it can end inside a hexadecimal digit or group. The calculator applies that bit boundary when producing the network and last address.

IP Prefix Calculator: IPv6 Subnet Calculation

Entering an IPv6 Address and Prefix

Enter an IPv6 address followed by a prefix length, such as 2001:db8:1234:5678::9/56. You can also enter the address alone and choose Current prefix length from the list. Select Calculate Prefix to find the containing network, its first and last addresses, and its total size.

The input can be a host address within the prefix; it does not have to be the network's first address. The calculator keeps the network bits and sets the remaining bits to zero to find the start of the block. The displayed network address can therefore differ from the address you entered.

Current prefix lengths from /0 through /128 are accepted. If you enter a prefix after the address and select one from the list, the two values must match. A conflicting pair returns an error. Use a numeric IPv6 address without a URL, port, brackets, or zone identifier.

First and Last Addresses

The first and last addresses mark the boundaries of the entire prefix. These values do not indicate which addresses can be assigned to devices. IPv6 does not have broadcast addresses, so the last address is not a broadcast address.

For the example 2001:db8:1234:5678::9/56, the network is 2001:db8:1234:5600::/56. Its last address is 2001:db8:1234:56ff:ffff:ffff:ffff:ffff. The entered address lies between those boundaries. The 2001:db8::/32 range is reserved for documentation, so this example is for calculation rather than use on a public network.

The result's address type and scope describe the address you entered. Before assigning addresses, confirm that the block is allocated to you and that your chosen values are not reserved for another purpose. The calculator does not assign addresses or check which ones are already in use.

How the Address Count Is Calculated

An IPv6 address contains 128 bits. A prefix length of p leaves 128 minus p address bits, giving 2 raised to that power as the total number of addresses. The result displays the complete decimal count rather than a rounded estimate.

A /64 contains 18,446,744,073,709,551,616 addresses. A larger slash number reduces that count; a /65 has half as many. The count describes the number of addresses in the block, not how many devices a network can support in practice.

For a current prefix no longer than /64, the result also shows how many /64 prefixes it contains. A /56 contains 256, while a /48 contains 65,536. At the other end of the size range, a /127 contains two addresses and a /128 contains one.

Dividing a Block into Smaller Prefixes

Choose Subnet prefix length only when you want a subnet list. Leave the selection at No subnetting to calculate the current block alone. A child prefix must be numerically longer than the current prefix and no longer than /128. A /128 cannot be divided further.

The difference between the two prefix lengths determines the number of child blocks. Dividing a /56 into /60 prefixes uses four additional network bits and produces 16 equal subnets. Each child block contains 16 /64 prefixes. Together, the child blocks cover the original block without extending beyond its boundaries.

For 2001:db8:1234:5600::/56, the /60 list begins with 2001:db8:1234:5600::/60, followed by 2001:db8:1234:5610::/60. The final child is 2001:db8:1234:56f0::/60. Each new subnet starts one whole /60 block after the previous one, so incrementing the last digit of an address would not produce the same sequence.

Expanded Addresses and Reverse DNS

The detail fields show the entered IPv6 address, its expanded form, and the expanded network prefix. The expanded values make it easier to compare the input with the calculated boundary. Network bits and Address bits show how the 128-bit value was divided for this calculation.

The reverse DNS zone is derived from the prefix's hexadecimal digits. A single zone name can be shown when the prefix length is a multiple of four. For other lengths, the tool states that no single zone name is available.

The Reverse DNS zone field gives a name based on the calculated prefix; it does not query a DNS server or create a reverse delegation. A correctly formed zone name does not show that you control the zone or that any reverse records have been published.

Copying and Browsing the Subnet List

The browser displays the calculated child prefixes in pages of up to 4,096 entries. Use Previous Prefixes and Next Prefixes to move through a longer list. The list contains prefixes, not every individual IPv6 address inside each prefix.

Copy All IPv6 Prefixes copies the entire subnet list when it contains no more than 4,096 entries. For a larger list, use Copy Displayed Prefixes to copy the current page. Copy Calculation Results separately copies the boundary, capacity, and detail fields.

JavaScript must be enabled to display and copy the interactive subnet list. If JavaScript is disabled, you can still calculate the network boundaries and address counts, but the list controls are unavailable.