More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /124 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 /124
- Prefix mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:fff0
- Total addresses: 2⁴
- Approx. count: 16
- /64 subnets: —
IPv6 /124 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
netnetnetnetnetnetnetmix
Network bits (124)Split hextetHost bits (4)
- Network bits
- 124
- Host bits
- 4
- Prefix mask
- ffff:ffff:ffff:ffff:ffff:ffff:ffff:fff0
- Total addresses
- 2⁴
- Approx. count
- 16
- /64 subnets
- —
- Addresses formula
- 2^4
- /64 relationship
- smaller than a /64
Overview
A /124 leaves 4 host bits, giving exactly 16 addresses. It is used for very small point-to-point or stub networks where even a /120 would be larger than necessary.
Common use cases
- Tiny point-to-point or stub links
- Lab exercises in subnetting
- Constrained test environments
Key facts
- A /124 fixes 124 network bits and leaves 4 host bits — 2⁴ total addresses.
- In network design terms, /124 is typically a legacy point-to-point size.
- Written out, /124 holds exactly 16 addresses.
- A /124 is 1/2⁶⁰ of a standard /64 LAN subnet.
Design guidance
A /124 still works on point-to-point links but wastes addresses compared to /127. If you inherit a legacy /124 design, it will function; for new builds, standardize on /127 per RFC 6164 to eliminate neighbor-discovery attack surface.
Practical example
In a lab, 2001:db8:abcd:0012::1/124 might number a small segment with 16 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 /124 blocks, prefix sizes, and use cases.