Why does every LAN need a /64?

SLAAC builds a 64-bit interface identifier, and RFC 4291 fixes the interface ID at 64 bits for unicast addresses outside ::/3. Prefixes longer than /64 on a LAN break autoconfiguration.

Should point-to-point links use /64 or /127?

RFC 6164 recommends /127 on inter-router links to prevent neighbor-cache exhaustion attacks. Many operators allocate a /64 per link and configure a /127 inside it.

What is 2001:db8::/32?

The documentation prefix (RFC 3849). Use it in examples and labs, never in production.

IPv6 Subnet Calculator: Split Prefixes into /64s

IPv6 subnetting is counting bits

There is no broadcast and no reserved host math in IPv6 planning. The number of subnets of length n inside a prefix of length p is simply:

subnets = 2^{(n − p)}

From a /48 to /64 you borrow 16 bits, so you get 2¹⁶ = 65,536 subnets. Each /64 holds 2⁶⁴ interface addresses, which is why you never size an IPv6 LAN by host count.

Standard prefix sizes

Prefix	Typical use	/64s inside
/32	Minimum LIR/ISP allocation from an RIR	4,294,967,296
/48	Enterprise site, PI space from an RIR	65,536
/56	Residential or small business delegation	256
/60	Minimal delegation used by some ISPs	16
/64	One LAN segment (required for SLAAC)	1
/127	Point-to-point link (RFC 6164)	–
/128	Loopback, single host	–

Worked example: 2001:db8:abcd::/48 into /64s

The fourth hextet (16 bits) becomes your subnet ID. The tool lists them in order:

2001:db8:abcd::/64
2001:db8:abcd:1::/64
2001:db8:abcd:2::/64
...
2001:db8:abcd:ffff::/64

Notation follows RFC 5952: lowercase hex, leading zeros dropped, and the longest run of zero hextets replaced by ::. The expanded row shows all 32 hex digits, which is what you need for reverse DNS (ip6.arpa) zones.

Plan on nibble boundaries

Each hex digit is 4 bits (a nibble). Prefix lengths that are multiples of 4 (/48, /52, /56, /60, /64) keep subnet boundaries aligned with hex digits, so a subnet ID is readable directly from the address and reverse DNS delegation stays clean. A common enterprise layout for a /48:

First nibble of the 4th hextet = region or building (16 values).
Second nibble = function (users, voice, servers, management…).
Last two nibbles = VLAN or segment number (256 values).

Configuration

! Cisco IOS
interface Vlan20
 ipv6 address 2001:db8:abcd:20::1/64

# Junos
set interfaces irb unit 20 family inet6 address 2001:db8:abcd:20::1/64

# Linux
ip -6 addr add 2001:db8:abcd:20::1/64 dev eth0

Rules of thumb

Every LAN gets a /64 (RFC 4291 interface IDs are 64 bits). Smaller prefixes break SLAAC and several other features.
Use /127 on router-to-router links (RFC 6164), but reserve a full /64 per link in your plan so you can change your mind later.
Do not try to conserve space; plan for structure and summarization instead.

IPv6 subnet calculator