IPv6 /47 Prefix ReferenceIPv6
The pair-broker of IPv6: a /47 is precisely two adjacent /48s, and its main claim to fame is letting you announce two site blocks as one route. Beyond that trick it is an odd duck — 131,072 /64 subnets wearing a prefix found on no standard catalog page.
/47 facts at a glance
- Total addresses
- 2,417,851,639,229,258,349,412,352 (281)
- /64 subnets inside
- 131,072
- Host/interface bits
- 81
- Example network
- 2001:db8::/47
- First address
- 2001:db8::
- Last address
- 2001:db8:1:ffff:ffff:ffff:ffff:ffff
- Typical role
- Unusual split of a /46 or /48 region
A /47 in documentation space
Using 2001:db8::/32 (RFC 3849 documentation space) aligned to its own boundary, a /47 occupies:
| Block | First address | Last address |
|---|---|---|
| 2001:db8::/47 | 2001:db8:: | 2001:db8:1:ffff:ffff:ffff:ffff:ffff |
How an IPv6 /47 prefix works
A /47 prefix pins the first 47 bits of a 128-bit address, leaving 81 bits free. That free portion holds 281 = 2,417,851,639,229,258,349,412,352 distinct addresses, and every such block begins at a multiple of that size. Because each subnet should be a /64, the 17 middle bits between the prefix and the /64 boundary give 131,072 subnets to hand out.
Where /47 is used
- Aggregating two neighboring /48s into a single upstream announcement.
- Halving a /46 during top-down subdivision of a larger grant.
- Edge cases in addressing plans where two site blocks share one fate.
Arithmetic first, romance second: two /48s, four /49s, eight /50s. If your design leans on /47s, double-check that the halves really belong together.
Because /47 falls off the nibble grid, expect it to appear as internal scaffolding far more often than as an official assignment.
Frequently asked questions
How many /48s are in a /47?
Two — that is the entire identity of the prefix. A single bit separates /47 from the classic site size.
How many addresses does an IPv6 /47 have?
2,417,851,639,229,258,349,412,352, which is 2^81.
Should I announce a /47 instead of two /48s?
If both blocks travel the same path, one /47 announcement trims the routing table; keep the /48s routable too so the pieces survive deaggregation.
How many /64 subnets live in a /47?
131,072. Seventeen bits of /64 space, split evenly across the two embedded /48s.
Open this prefix in the calculator
Open 2001:db8::/47 in the IPv6 calculator