More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /40 subnets
- Expanded address
- Compressed address
- Network (expanded)
- Last address (expanded)
- Prefix mask
- Total addresses (exact)
- Reverse DNS (PTR)
- Host bits / network bits
Hextet breakdown
Quick facts for IPv6 /40
- Prefix mask: ffff:ffff:ff00::
- Total addresses: 2⁸⁸
- Approx. count: 3.09 × 10²⁶
- /64 subnets: 2²⁴
IPv6 /40 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
- Network bits
- 40
- Host bits
- 88
- Prefix mask
- ffff:ffff:ff00::
- Total addresses
- 2⁸⁸
- Approx. count
- 3.09 × 10²⁶
- /64 subnets
- 2²⁴
- Addresses formula
- 2^88
- /64 relationship
- 2²⁴ × /64 subnets
Overview
A /40 is an intermediate allocation, larger than the common /48 site but smaller than an ISP /32. It holds 2⁸⁸ addresses — 256 /48 sites — and is sometimes assigned to large enterprises or smaller ISPs.
Common use cases
- Large enterprise allocations
- Smaller ISP or regional provider blocks
Key facts
- A /40 fixes 40 network bits and leaves 88 host bits — 2⁸⁸ total addresses.
- In network design terms, /40 is typically a enterprise or multi-site allocation.
- You can subnet a /40 into about 2²⁴ /64 LANs.
- At site scale, /40 equals 2⁸ /48 allocations.
Design guidance
A /40 suits organizations that have outgrown a single /48 but do not need a full /32 ISP allocation. Plan your addressing scheme before delegating: assign one /48 (or smaller) per major site, then subnet each site into /64 LANs. Document your nibble boundaries so future growth does not force renumbering.
Practical example
An ISP holding 2001:db8:abcd::/40 might announce the entire /40 to upstream providers as one BGP route, then delegate /48 blocks such as 2001:db8:0001::/48 and 2001:db8:0002::/48 to business customers. Each customer subnets their /48 into /64 LANs.
Related RFCs and standards
- RFC 4291IPv6 Addressing Architecture
Prefix sizing reference
Frequently asked questions
Common questions about IPv6 /40 blocks, prefix sizes, and use cases.