IPv6 calculator

An IPv6 address has 128 bits, written as eight groups of four hexadecimal digits separated by colons: 2001:0db8:0000:0000:0000:ff00:0042:8329. Two rules make it shorter:

• Leading zeros in each group can be dropped: 0db8 becomes db8 and 0042 becomes 42.
• One run of consecutive all-zero groups can be replaced with “::”.

So the address above becomes 2001:db8::ff00:42:8329. The calculator shows both forms so you can check.

IPv6 has no dotted decimal masks: it always uses a prefix, such as /48 or /64. The rule is that every local network (a VLAN, a Wi-Fi network) is a /64, because mechanisms such as SLAAC autoconfiguration require it. Providers usually give each customer a /56 or a /48, and the /64s come out of that.

• A /48 contains 65,536 /64 subnets.
• A /56 contains 256 /64 subnets.
• A /60 contains 16 /64 subnets.

Each nibble (one hexadecimal digit) is 4 bits, so the easiest plans split on multiples of 4: /48, /52, /56, /60 and /64.

• Global unicast (2000::/3): public addresses, routable on the internet.
• Unique local or ULA (fc00::/7): the equivalent of IPv4 private addresses.
• Link-local (fe80::/10): only valid on the same segment; every host has one.
• Multicast (ff00::/8): for sending to a group of hosts. IPv6 has no broadcast.
• Documentation (2001:db8::/32): reserved for examples, like the ones on this page.

Suppose your provider assigns you the prefix 2001:db8:abcd::/48. A tidy way to split it is to use the fourth group (16 bits) to number your networks:

• 2001:db8:abcd:0000::/64 to 2001:db8:abcd:00ff::/64 for headquarters (256 networks).
• 2001:db8:abcd:0100::/56 for the first branch, 2001:db8:abcd:0200::/56 for the second, and so on up to 256 sites.
• Inside each /56, one /64 per VLAN: office, Wi-Fi, servers, phones…

By aligning the splits with hexadecimal digits, every address “reads” on its own: the fourth group tells you which site and which network it belongs to. In the calculator, choose the /48 prefix and split into /56 or /64 to see the plan.