More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /126 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 /126
- Prefix mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:fffc
- Total addresses: 2²
- Approx. count: 4
- /64 subnets: —
IPv6 /126 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
netnetnetnetnetnetnetmix
Network bits (126)Split hextetHost bits (2)
- Network bits
- 126
- Host bits
- 2
- Prefix mask
- ffff:ffff:ffff:ffff:ffff:ffff:ffff:fffc
- Total addresses
- 2²
- Approx. count
- 4
- /64 subnets
- —
- Addresses formula
- 2^2
- /64 relationship
- smaller than a /64
Overview
A /126 provides 4 addresses, comparable to an IPv4 /30. It was historically used for router-to-router point-to-point links before /127 became the recommended choice.
Common use cases
- Legacy point-to-point router links
- Migration from IPv4 /30 thinking
Key facts
- A /126 fixes 126 network bits and leaves 2 host bits — 2² total addresses.
- In network design terms, /126 is typically a legacy point-to-point size.
- Written out, /126 holds exactly 4 addresses.
- A /126 is 1/2⁶² of a standard /64 LAN subnet.
Design guidance
A /126 still works on point-to-point links but wastes addresses compared to /127. If you inherit a legacy /126 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::1/126 might number a small segment with 4 addresses. In production, you would normally expand this to a full /64 unless you have a documented exception.
Related RFCs and standards
- RFC 6164Using 127-Bit IPv6 Prefixes on Inter-Router Links
Prefix sizing reference
Frequently asked questions
Common questions about IPv6 /126 blocks, prefix sizes, and use cases.