IPv6 Cheat Sheet

The IPv6 basics you need every day: how an address is built, how to shorten it, how big each prefix is, and the ranges to recognise.

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IPv6 Cheat Sheet

Anatomy of an address

2001:0db8:abcd:0012:0000:0000:0000:0001

48 bits: site prefix · 16: subnet · 64: interface ID

Shorten in 2 steps

2001:0db8:0000:0000:0000:ff00:0042:8329

1drop leading 0s in each group

2001:db8:0:0:0:ff00:42:8329

2longest run of 0 groups → ::

2001:db8::ff00:42:8329

:: only ONCE · lower case · one 0 group stays "0"

Prefix sizes

Prefix	For	Holds
/32	ISP / LIR	65,536 × /48
/48	a site	65,536 × /64
/56	a home	256 × /64
/64	one LAN	2^64 hosts
/127	p2p link	2 addresses
/128	one host	1 address

count /64s = 2^(64 − prefix)

Golden rule every LAN is a /64
SLAAC needs it. Don't go smaller.

Subnet a /48

Site: 2001:db8:abcd::/48

4th group = subnet ID (0–ffff)

VLAN 18 → hex 0012 →

2001:db8:abcd:12::/64

Ranges to know

::1 loopback · :: unspecified

fe80::/10 link-local

fc00::/7 unique local (fd…)

ff00::/8 multicast · ff02::1 all

2000::/3 global unicast

2001:db8::/32 docs only

::ffff:0:0/96 IPv4-mapped

no broadcast in IPv6, only multicast ✎subnetcalculator.dev

Common IPv6 mistakes

Dropping trailing zeros. Only leading zeros can go: 0db8 becomes db8, but ff00 stays ff00.
Giving a LAN a /80 or /96 "to save space". It breaks SLAAC and saves nothing: a /48 already holds 65,536 LANs.
Forgetting the subnet ID is hex. VLAN 18 is :12:, VLAN 100 is :64:. Some teams use the decimal digits instead (:100:) for readability; pick one and document it.

FAQ

Why is every IPv6 LAN a /64?

SLAAC (stateless address autoconfiguration) builds the last 64 bits of the address from the device itself, so the network part has to be exactly 64 bits. Many other features assume /64 too. Use longer prefixes only for point-to-point links (/127) and loopbacks (/128).

Can I use :: twice in one address?

No. :: stands for 'as many zero groups as needed to make 8', so two of them would be ambiguous. Compress the longest run of zero groups; if two runs are the same length, compress the first one.

Does IPv6 have a broadcast address?

No. IPv6 uses multicast instead, for example ff02::1 for all nodes on the link. That is also why there is no 'minus 2' when counting usable addresses in an IPv6 subnet.

Tools: IPv6 compression · IPv6 subnet calculator · MAC to IPv6 · More sheets: subnet · wildcard mask