IPv6 prefixes, without the oversized guesswork.
Read compressed addresses, count a prefix exactly, and split an IPv6 allocation into subnets.
128 bits, eight hexadecimal groups
IPv6 addresses contain eight 16-bit groups, usually written in hexadecimal. Leading zeros inside a group can be omitted. A double colon can replace a sequence of zero groups, but may appear only once. The calculator accepts both full and compressed forms, including an IPv4 tail such as ::ffff:192.0.2.1.
The compressed hexadecimal output uses lowercase, removes leading zeros, and compresses the longest run of at least two zero groups. A tie uses the leftmost run. For recognized IPv4-mapped addresses, the calculator also shows the mixed decimal-tail form.
A count is not an assignment policy
A /64 contains 2^64 addresses: 18,446,744,073,709,551,616. JavaScript floating-point numbers cannot represent every integer of this size. SubnetTidy uses BigInt for address arithmetic and writes large numbers as decimal strings in JSON so the export does not lose precision.
IPv6 has no broadcast address. It is incorrect to copy the IPv4 “subtract two” formula. The calculator reports the total size and boundaries of the prefix. Interface identifiers, anycast, address scope and deployment rules still determine what should be assigned.
Split a /48 into /64 networks
A /48 allocation contains 2^(64 − 48), or 65,536, distinct /64 prefixes. Enter 2001:db8:1234::/48 into the splitter and choose 64. The result begins with 2001:db8:1234::/64 and 2001:db8:1234:1::/64.
The interface shows 32 subnets at a time and keeps the total exact, even for very large spaces. It never constructs a list of every individual address. The 2001:db8::/32 examples are documentation addresses, not an allocation for public deployment.
Keep syntax separate from connectivity
Do not include brackets, port numbers or interface zone identifiers in the address field. For example, use fe80::1/64 rather than [fe80::1]:443 or fe80::1%eth0. A plain IPv6 address is treated as /128.
The tool does not query DNS, scan a host or verify network reachability. A valid address string and correct arithmetic do not establish that the address is configured or routed. Use your network documentation to choose operational prefixes.
Put it into practice
RFC 3021 · IPv4 /31 · RFC 4291 · IPv6 · RFC 5952 · IPv6 text