IPv6 Subnet & Address Compression Calculator

Compress or expand 128-bit IPv6 addresses per RFC 5952, calculate prefix boundaries, count available /64 SLAAC subnets, and generate reverse DNS (ip6.arpa) zones.

IPv6 Prefix & Address Details

How IPv6 Addressing, RFC 5952 Compression & /64 Subnetting Work

IPv6 expands the IP address space from 32 bits to 128 bits, written as eight 16-bit hexadecimal groups (hextets) separated by colons. Under RFC 5952 canonical text rules, leading zeros inside each hextet are omitted (0370 becomes 370), hexadecimal letters are written in lowercase, and the single longest contiguous run of two or more all-zero hextets is collapsed into a double colon (::).

Unlike IPv4—where administrators squeeze host counts into tight /27 or /28 blocks—IPv6 architecture standardizes almost every end-user LAN, VLAN, and cloud VPC segment on a fixed /64 prefix. Stateless Address Autoconfiguration (SLAAC, RFC 4862) requires a 64-bit interface identifier, meaning prefixes longer than /64 break standard neighbor discovery features on many operating systems. Point-to-point router links typically allocate a /64 while configuring /127 (RFC 6164) on the interfaces themselves to mitigate Neighbor Discovery cache exhaustion attacks.

ISPs and enterprise registries commonly delegate a /48 block per site (providing 65,536 distinct /64 subnets) or a /56 block for smaller branches and home routers (providing 256 distinct /64 subnets). Reverse DNS PTR delegations in ip6.arpa reverse every 4-bit hexadecimal nibble up to the prefix boundary.