More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /120 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 /120
- Prefix mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:ff00
- Total addresses: 2⁸
- Approx. count: 256
- /64 subnets: —
IPv6 /120 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
netnetnetnetnetnetnetmix
Network bits (120)Split hextetHost bits (8)
- Network bits
- 120
- Host bits
- 8
- Prefix mask
- ffff:ffff:ffff:ffff:ffff:ffff:ffff:ff00
- Total addresses
- 2⁸
- Approx. count
- 256
- /64 subnets
- —
- Addresses formula
- 2^8
- /64 relationship
- smaller than a /64
Overview
A /120 leaves 8 host bits, giving exactly 256 addresses. It is used for small, tightly scoped segments — such as a single VLAN or management subnet — where a full /64 would waste address space.
Common use cases
- Small management or out-of-band networks
- Lab subnets with a fixed host count
- Constrained VLAN segments
Key facts
- A /120 fixes 120 network bits and leaves 8 host bits — 2⁸ total addresses.
- In network design terms, /120 is typically a constrained management or lab segment.
- Written out, /120 holds exactly 256 addresses.
- A /120 is 1/2⁵⁶ of a standard /64 LAN subnet.
Design guidance
A /120 can work for small management VLANs, out-of-band networks, or certification lab exercises where you deliberately cap the host count. For router interconnects, prefer /127 (RFC 6164) over /120. For ordinary LANs, stay with /64 regardless of how small the segment feels.
Practical example
In a lab, 2001:db8:abcd:0012::1/120 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 /120 blocks, prefix sizes, and use cases.