More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /72 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 /72
- Prefix mask: ffff:ffff:ffff:ffff:ff00::
- Total addresses: 2⁵⁶
- Approx. count: 7.21 × 10¹⁶
- /64 subnets: —
IPv6 /72 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
netnetnetnetmixhosthosthost
Network bits (72)Split hextetHost bits (56)
- Network bits
- 72
- Host bits
- 56
- Prefix mask
- ffff:ffff:ffff:ffff:ff00::
- Total addresses
- 2⁵⁶
- Approx. count
- 7.21 × 10¹⁶
- /64 subnets
- —
- Addresses formula
- 2^56
- /64 relationship
- smaller than a /64
Overview
A /72 is a sub-/64 block covering 1/2⁸ of a standard LAN subnet — 2⁵⁶ addresses. Using prefixes smaller than /64 on a LAN breaks SLAAC and is discouraged except in controlled lab environments.
Common use cases
- Lab exercises on sub-/64 addressing
- Documentation of non-standard prefix lengths
- Controlled environments without SLAAC
Key facts
- A /72 fixes 72 network bits and leaves 56 host bits — 2⁵⁶ total addresses.
- In network design terms, /72 is typically a specialized sub-/64 block.
- A /72 is 1/2⁸ of a standard /64 LAN subnet.
Design guidance
Avoid /72 on production LANs — it breaks SLAAC on most hosts. If you are experimenting in a lab, document why you chose a sub-/64 and ensure all devices support static addressing or DHCPv6. For real networks, assign a full /64 per segment instead of carving 1/2⁸ of one.
Practical example
In a lab, 2001:db8:abcd:0012::1/72 might number a small segment with 2⁵⁶ 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 /72 blocks, prefix sizes, and use cases.