More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /91 subnets
- Expanded address
- Compressed address
- Network (expanded)
- Last address (expanded)
- Prefix mask
- Total addresses (exact)
- Reverse DNS (PTR)
- Host bits / network bits
Hextet breakdown
20010db8abcd00120000000000000001
NetworkSplit groupHost
Quick facts for IPv6 /91
- Prefix mask: ffff:ffff:ffff:ffff:ffff:ffe0::
- Total addresses: 2³⁷
- Approx. count: 1.37 × 10¹¹
- /64 subnets: —
IPv6 /91 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
netnetnetnetnetmixhosthost
Network bits (91)Split hextetHost bits (37)
- Network bits
- 91
- Host bits
- 37
- Prefix mask
- ffff:ffff:ffff:ffff:ffff:ffe0::
- Total addresses
- 2³⁷
- Approx. count
- 1.37 × 10¹¹
- /64 subnets
- —
- Addresses formula
- 2^37
- /64 relationship
- smaller than a /64
Overview
A /91 leaves 37 host bits (2³⁷ addresses). It is smaller than a /64 LAN and appears in specialized schemes — for example /96 embeds a 32-bit IPv4 address for NAT64 or IPv4-mapped addressing.
Common use cases
- NAT64 and IPv4-embedded addressing study
- Specialized translation gateway design
- Protocol documentation and examples
Key facts
- A /91 fixes 91 network bits and leaves 37 host bits — 2³⁷ total addresses.
- In network design terms, /91 is typically a specialized sub-/64 block.
- A /91 is 1/2²⁷ of a standard /64 LAN subnet.
Design guidance
A /91 is a specialized size, not a general LAN substitute. Production VLANs should remain /64. Use /91 only when a protocol or design explicitly calls for it — for example /96 in NAT64 translation — or in controlled lab environments.
Practical example
In a lab, 2001:db8:abcd:0012::1/91 might number a small segment with 2³⁷ addresses. In production, you would normally expand this to a full /64 unless you have a documented exception.
Prefix sizing reference
Frequently asked questions
Common questions about IPv6 /91 blocks, prefix sizes, and use cases.