IPv6 Subnet Calculator

IPv4 addresses are 32 bits long, which gives 2^32 (about 4.29 billion) total addresses. IPv6 expanded that to 128 bits, which is 2^128 (about 3.4 x 10^38) total addresses. That is a 79-octillion-fold increase, enough to assign trillions of unique addresses to every grain of sand on Earth. The bigger address means IPv6 is written as 8 groups of 4 hex digits separated by colons (for example 2001:0db8:85a3:0000:0000:8a2e:0370:7334) instead of the four decimal octets of IPv4.

A /64 is the normal size for an IPv6 LAN because SLAAC uses a 64-bit interface identifier. Prefixes longer than /64 generally cannot use ordinary SLAAC, but Neighbor Discovery and DHCPv6 are not universally broken by that fact. Other link types have specific guidance, including /127 for many router-to-router links.

Two rules. First, leading zeros inside any 16-bit group can be dropped (2001:0db8 and 2001:db8 are the same). Second, exactly one run of consecutive all-zero groups can be replaced with :: (2001:db8:0:0:0:0:0:1 becomes 2001:db8::1). The :: shorthand may appear at most once per address, otherwise the parser would not know how many zeros to insert on each side. RFC 5952 also picks the longest zero run and breaks ties toward the leftmost, which is what canonical tools produce.

Common ranges include global unicast 2000::/3, link-local fe80::/10, unique-local fc00::/7, and multicast ff00::/8. Locally assigned unique-local prefixes currently use fd00::/8; fc00::/8 is reserved for another assignment method. IPv6 also defines loopback ::1, unspecified ::, IPv4-mapped addresses, and the documentation prefix 2001:db8::/32.

A /64 leaves 64 interface-identifier bits, so it contains 2^64, or 18,446,744,073,709,551,616, addresses. IPv6 plans normally assign /64 to SLAAC LANs for protocol consistency rather than trying to fill the address space. Site allocations vary by provider and policy, so /48 and /56 are common examples, not universal entitlements.