IPv6 network details
Address count is the total size of the prefix, not a promise that every address can be assigned to a host. Values can be copied individually.
Understand this calculator
IPv6 addresses contain 128 bits and are commonly written as eight hexadecimal groups separated by colons. A CIDR prefix tells you how many leading bits identify the network. This tool expands and compresses addresses and calculates the range covered by a prefix.
How to use this calculator
- IPv6 address
- Enter a valid IPv6 address, such as 2001:db8:abcd:12::1. A double colon (::) can compress one continuous run of zero groups.
- CIDR prefix
- Enter an integer from 0 to 128. A /64 fixes the first 64 bits as the network portion and leaves 64 bits in the address range.
Formula and method
IPv6 has 128 bits. A /64 therefore contains 2^64 = 18,446,744,073,709,551,616 possible addresses. These are addresses in the prefix, not a promise that every address can be assigned to a host.
Worked example: 2001:db8:abcd:12::1/64
The expanded address is 2001:0db8:abcd:0012:0000:0000:0000:0001. With /64, the network is 2001:db8:abcd:12::/64. The first address in the prefix is 2001:0db8:abcd:0012:0000:0000:0000:0000, and the last is 2001:0db8:abcd:0012:ffff:ffff:ffff:ffff. The prefix contains exactly 18,446,744,073,709,551,616 addresses.
Understanding the results
- Compressed and expanded address
- Compressed notation omits leading zeros and may use :: for one run of zero groups. Expanded notation shows all eight four-digit hexadecimal groups.
- Network prefix
- The network address is obtained by clearing all bits beyond the specified prefix length.
- First and last address
- These are the numeric boundaries of the selected prefix. IPv6 does not use IPv4-style broadcast addresses.
Assumptions and common mistakes
- Address count versus usable hosts
- A /64 contains 2^64 addresses, but network design and address-assignment rules determine practical usage.
- IPv6 compression
- Only one :: is allowed in a single IPv6 address; it represents one or more omitted zero groups.
- Documentation addresses
- 2001:db8::/32 is reserved for examples and documentation, not real public deployment.
Frequently asked questions
What is the difference between /64 and /128?
A /64 leaves 64 variable address bits; a /128 identifies exactly one IPv6 address.
Does IPv6 have a broadcast address?
No. IPv6 uses multicast and other mechanisms rather than IPv4 broadcast.
Why are there so many addresses in a /64?
Because 64 host bits permit 2^64 unique bit combinations.