IPv6 /40 Prefix ReferenceIPv6
Wholesale arithmetic for hosting providers: a /40 is 256 classic site allocations deep, wrapping more than a quarter-billion /64 subnets behind one prefix. Secure one of these and you can put a /48 behind every customer cage you operate.
/40 facts at a glance
- Total addresses
- 309,485,009,821,345,068,724,781,056 (288)
- /64 subnets inside
- 16,777,216
- Host/interface bits
- 88
- Example network
- 2001:db8::/40
- First address
- 2001:db8::
- Last address
- 2001:db8:ff:ffff:ffff:ffff:ffff:ffff
- Typical role
- Growing ISP or multi-site enterprise
A /40 in documentation space
Using 2001:db8::/32 (RFC 3849 documentation space) aligned to its own boundary, a /40 occupies:
| Block | First address | Last address |
|---|---|---|
| 2001:db8::/40 | 2001:db8:: | 2001:db8:ff:ffff:ffff:ffff:ffff:ffff |
How an IPv6 /40 prefix works
A /40 prefix pins the first 40 bits of a 128-bit address, leaving 88 bits free. That free portion holds 288 = 309,485,009,821,345,068,724,781,056 distinct addresses, and every such block begins at a multiple of that size. Because each subnet should be a /64, the 24 middle bits between the prefix and the /64 boundary give 16,777,216 subnets to hand out.
Where /40 is used
- Hosting providers stamping out one /48 per customer or per rack.
- Large enterprises giving every business unit its own site-sized slice.
- Cloud regions pooling address space ahead of finer delegation.
A /40 descends cleanly into sixteen /44s or 256 /48s, keeping every boundary nibble-aligned for readable reverse DNS zones.
At 16,777,216 /64s, exhaustion is never the operational concern; aggregation discipline is.
Frequently asked questions
How many /48s are in a /40?
Exactly 256. Eight bits separate the prefixes, which is why the /40 is beloved wherever customers are billed by the site block.
How many /64 subnets does a /40 contain?
16,777,216 — twenty-four bits' worth of standard subnets.
How many addresses does an IPv6 /40 have?
309,485,009,821,345,068,724,781,056, which works out to 2^88.
Is a /40 a realistic enterprise assignment?
For very large organizations, yes; more often it functions as the provider-side container from which enterprise /44s, /48s, and /56s are carved.
Open this prefix in the calculator
Open 2001:db8::/40 in the IPv6 calculator