/48 → /56 → /64 · RFC 6177 · SLAAC

IPv6 Subnet Plan

Q: How many /64 subnets are in a /48?

65,536. A /48 leaves 16 bits before the /64 boundary, and 2^16 = 65,536. Splitting the same /48 into /56 sites gives 256 sites with 256 /64s each.

Split a parent prefix into site blocks and then into LAN blocks. The default plan takes one /48, gives each site a /56 (256 /64 LANs) and lists the first sites with their first and last /64.

Hierarchy

parent · site · LAN

Parent prefix

Site prefix

LAN prefix

Try

Hierarchy2001:db8:abcd::/48 → /56 sites → /64 LANs

Sites256 /56 blocks (2⁸)

LANs per site256 /64 blocks (2⁸)

Total /64 LANs65,536 (2¹⁶)

Site	/64s	First /64	Last /64
2001:db8:abcd::/56	256	2001:db8:abcd::/64	2001:db8:abcd:ff::/64
2001:db8:abcd:100::/56	256	2001:db8:abcd:100::/64	2001:db8:abcd:1ff::/64
2001:db8:abcd:200::/56	256	2001:db8:abcd:200::/64	2001:db8:abcd:2ff::/64
2001:db8:abcd:300::/56	256	2001:db8:abcd:300::/64	2001:db8:abcd:3ff::/64
2001:db8:abcd:400::/56	256	2001:db8:abcd:400::/64	2001:db8:abcd:4ff::/64
2001:db8:abcd:500::/56	256	2001:db8:abcd:500::/64	2001:db8:abcd:5ff::/64
2001:db8:abcd:600::/56	256	2001:db8:abcd:600::/64	2001:db8:abcd:6ff::/64
2001:db8:abcd:700::/56	256	2001:db8:abcd:700::/64	2001:db8:abcd:7ff::/64
2001:db8:abcd:800::/56	256	2001:db8:abcd:800::/64	2001:db8:abcd:8ff::/64
2001:db8:abcd:900::/56	256	2001:db8:abcd:900::/64	2001:db8:abcd:9ff::/64
2001:db8:abcd:a00::/56	256	2001:db8:abcd:a00::/64	2001:db8:abcd:aff::/64
2001:db8:abcd:b00::/56	256	2001:db8:abcd:b00::/64	2001:db8:abcd:bff::/64
2001:db8:abcd:c00::/56	256	2001:db8:abcd:c00::/64	2001:db8:abcd:cff::/64
2001:db8:abcd:d00::/56	256	2001:db8:abcd:d00::/64	2001:db8:abcd:dff::/64
2001:db8:abcd:e00::/56	256	2001:db8:abcd:e00::/64	2001:db8:abcd:eff::/64
2001:db8:abcd:f00::/56	256	2001:db8:abcd:f00::/64	2001:db8:abcd:fff::/64

Showing the first 16 of 256 sites. Last site: 2001:db8:abcd:ff00::/56.

How the hierarchy works

IPv6 planning is a chain of power-of-two splits. A /48 holds 16 bits before the /64 boundary, so it contains 2¹⁶ = 65,536 /64 blocks. Choosing a /56 site level divides those 16 bits into 8 for sites and 8 for LANs: 256 sites, each with 256 /64s. Choosing /52 instead gives 16 sites with 1,024 /64s each. The counts are exact powers of two, so a site block never straddles another site's space.

The parent is what you received: an ISP may hand a home a /56, an organisation a /48, a large customer a /40, or an ISP a /32.
The site level is one block per location, building or business unit.
The LAN level is one block per VLAN or segment. Keep it at /64: SLAAC and most IPv6 features assume a 64-bit interface ID.

RFC 6177 recommends handing end sites at least a /56 rather than a single /64, which is why the default here is /56 sites. For a worked walkthrough of the notation and the /64 rule, read IPv6 subnetting. To inspect one of the blocks, open it in the IPv6 tools, and for IPv4 planning with host counts use the VLSM planner. A fixed plan with every site listed is the /48 → /56 → /64 example.

Nibble boundaries and reverse DNS

Any prefix length routes correctly, but reverse DNS delegations move in 4-bit steps. A /48, /52, /56 or /60 site prefix maps to exactly one ip6.arpa zone; a /55 does not. The planner marks a level that is not on a nibble boundary so the choice is visible before you commit to it.

Questions

How many /64 subnets are in a /48?

65,536. A /48 leaves 16 bits before the /64 boundary, and 2¹⁶ = 65,536. Splitting the same /48 into /56 sites gives 256 sites with 256 /64s each.

Why use /56 for sites?

A /56 gives each site 256 /64 LANs, which is enough for most buildings or homes and keeps the prefix on a nibble boundary for reverse DNS delegation. RFC 6177 suggests /56 or /48 for end sites rather than a single /64.

Can I plan a /32 or /40 parent?

Yes. Change the parent, site and LAN prefix fields and the counts update. A /32 is typical for an ISP allocation, /40 for a large customer, /48 for one organisation and /56 for a small site.

Do prefix lengths need to be nibble boundaries?

Routing works with any prefix length, but reverse DNS zones delegate in 4-bit steps, so /48, /52, /56 and /60 map cleanly to one ip6.arpa zone. The planner notes when a chosen site or LAN prefix is not on a nibble boundary.