IPv6 /56 Prefix ReferenceIPv6
The default homework assignment from your ISP: a /56 hands a household 256 pristine /64 subnets. If your router advertises 'IPv6 prefix delegation', this is usually the block behind it.
/56 facts at a glance
- Total addresses
- 4,722,366,482,869,645,213,696 (272)
- /64 subnets inside
- 256
- Host/interface bits
- 72
- Example network
- 2001:db8::/56
- First address
- 2001:db8::
- Last address
- 2001:db8:0:ff:ffff:ffff:ffff:ffff
- Typical role
- Home network or department allocation
A /56 in documentation space
Using 2001:db8::/32 (RFC 3849 documentation space) aligned to its own boundary, a /56 occupies:
| Block | First address | Last address |
|---|---|---|
| 2001:db8::/56 | 2001:db8:: | 2001:db8:0:ff:ffff:ffff:ffff:ffff |
How an IPv6 /56 prefix works
A /56 prefix pins the first 56 bits of a 128-bit address, leaving 72 bits free. That free portion holds 272 = 4,722,366,482,869,645,213,696 distinct addresses, and every such block begins at a multiple of that size. Because each subnet should be a /64, the 8 middle bits between the prefix and the /64 boundary give 256 subnets to hand out.
Where /56 is used
- Residential and small-business delegation via DHCPv6 prefix delegation.
- Multi-subnet homes: IoT, guests, and work on separate /64 VLANs.
- Lab racks fed downstream subnets from a single upstream handout.
A /56 contains exactly 256 /64s, enough for one subnet per VLAN with plenty to spare, no follow-up request to the ISP required.
Frequently asked questions
How many /64s are in a /56?
Exactly 256. Eight delegation bits sit between /56 and /64.
Is a /56 enough for my home network?
Almost certainly: 256 subnets covers every VLAN a residence will ever plausibly run, with room for guests, lab gear, and future additions.
Can I subnet a /56 further than /64?
You can route longer prefixes manually, but hosts relying on SLAAC autoconfiguration expect a /64, so keep delegated LAN segments at exactly /64.
Why did my ISP give me a /56 instead of a /64?
A single /64 would lock you to one LAN. Delegating a container of subnets lets your router carve out a fresh /64 for each internal network.
Open this prefix in the calculator
Open 2001:db8::/56 in the IPv6 calculator