Worked example · IPv6 · nibble aligned

IPv6 /48 → /56 → /64 Example

Q: How many /64 LANs 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.

Q: What is the reverse DNS zone for this plan?

Write the /48 as nibbles, reverse them and add ip6.arpa: 2001:db8:acad::/48 becomes d.c.a.8.b.d.0.1.0.0.2.ip6.arpa. Each /56 or /64 site delegates its own zone from that point because every level is nibble aligned.

One /48 split into 256 /56 site blocks, each holding 256 /64 LANs. The table lists the first and last /64 of every site, so each site can be delegated as its own block.

Open this plan in the IPv6 planner IPv6 subnetting guide AWS example

The hierarchy

2001:db8:acad::/48 · 256 sites · 256 LANs per site

Hierarchy2001:db8:acad::/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:acad::/56	256	2001:db8:acad::/64	2001:db8:acad:ff::/64
2001:db8:acad:100::/56	256	2001:db8:acad:100::/64	2001:db8:acad:1ff::/64
2001:db8:acad:200::/56	256	2001:db8:acad:200::/64	2001:db8:acad:2ff::/64
2001:db8:acad:300::/56	256	2001:db8:acad:300::/64	2001:db8:acad:3ff::/64
2001:db8:acad:400::/56	256	2001:db8:acad:400::/64	2001:db8:acad:4ff::/64
2001:db8:acad:500::/56	256	2001:db8:acad:500::/64	2001:db8:acad:5ff::/64
2001:db8:acad:600::/56	256	2001:db8:acad:600::/64	2001:db8:acad:6ff::/64
2001:db8:acad:700::/56	256	2001:db8:acad:700::/64	2001:db8:acad:7ff::/64
2001:db8:acad:800::/56	256	2001:db8:acad:800::/64	2001:db8:acad:8ff::/64
2001:db8:acad:900::/56	256	2001:db8:acad:900::/64	2001:db8:acad:9ff::/64
2001:db8:acad:a00::/56	256	2001:db8:acad:a00::/64	2001:db8:acad:aff::/64
2001:db8:acad:b00::/56	256	2001:db8:acad:b00::/64	2001:db8:acad:bff::/64
2001:db8:acad:c00::/56	256	2001:db8:acad:c00::/64	2001:db8:acad:cff::/64
2001:db8:acad:d00::/56	256	2001:db8:acad:d00::/64	2001:db8:acad:dff::/64
2001:db8:acad:e00::/56	256	2001:db8:acad:e00::/64	2001:db8:acad:eff::/64
2001:db8:acad:f00::/56	256	2001:db8:acad:f00::/64	2001:db8:acad:fff::/64

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

How the split works

A /48 has 80 host bits, 16 of which are before the /64 boundary: 2¹⁶ = 65,536 /64 LANs. Choosing /56 for the site level splits those 16 bits into 8 for the site and 8 for the LAN, giving 256 sites of 256 LANs. Every boundary is a multiple of 4, so the plan reads directly from the hex digits:

2001:0db8:acad:0000:0000:0000:0000:0000   site 0, /56
2001:0db8:acad:0000:0000:0000:0000:0000   first /64 of site 0
2001:0db8:acad:00ff:0000:0000:0000:0000   last /64 of site 0
2001:0db8:acad:ff00:0000:0000:0000:0000   last site, /56

Site 0 is 2001:db8:acad::/56, holding 2001:db8:acad::/64 through 2001:db8:acad:ff::/64.
The last site is 2001:db8:acad:ff00::/56, holding the final 256 /64s up to 2001:db8:acad:ffff::/64.
Reverse DNS delegates in 4-bit steps: the /48 is one ip6.arpa zone, and each /56 or /64 can be delegated from it without splitting a nibble.

Ordinary LANs stay at /64 because SLAAC assumes a 64-bit interface ID (RFC 4862, analysed in RFC 7421). Router links can use /127 (RFC 6164) and loopbacks /128, but those are separate small blocks, not part of the site plan. RFC 6177 is the reason the site level here is /56 rather than handing each site a single /64.

Inspect a block in the IPv6 tools, or read the full walk-through in IPv6 subnetting. For IPv4 planning with host counts, see the AWS three-tier example.

Questions

How many /64 LANs 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 a site instead of /48?

RFC 6177 recommends giving an end site at least a /56 rather than a single /64. A /56 holds 256 /64 LANs, which fits most buildings and homes, and it stays on a nibble boundary for reverse DNS delegation.

Why must a LAN stay at /64?

SLAAC assumes a 64-bit interface ID, and RFC 7421 lists the features that break with other LAN sizes. Routed links can be /127 and loopbacks /128, but ordinary LANs are /64.

What is the reverse DNS zone for this plan?

Write the /48 as nibbles, reverse them and add ip6.arpa: 2001:db8:acad::/48 becomes d.c.a.8.b.d.0.1.0.0.2.ip6.arpa. Each /56 or /64 site delegates its own zone from that point because every level is nibble aligned.