More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /121 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 /121
- Prefix mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:ff80
- Total addresses: 2⁷
- Approx. count: 128
- /64 subnets: —
IPv6 /121 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
netnetnetnetnetnetnetmix
Network bits (121)Split hextetHost bits (7)
- Network bits
- 121
- Host bits
- 7
- Prefix mask
- ffff:ffff:ffff:ffff:ffff:ffff:ffff:ff80
- Total addresses
- 2⁷
- Approx. count
- 128
- /64 subnets
- —
- Addresses formula
- 2^7
- /64 relationship
- smaller than a /64
Overview
A /121 is a very small block with 2⁷ addresses (128) — comparable in spirit to an IPv4 point-to-point subnet. For router links, /127 (RFC 6164) is the modern best practice.
Common use cases
- Very small point-to-point or stub links
- Lab subnetting exercises
- Legacy designs migrating from IPv4 thinking
Key facts
- A /121 fixes 121 network bits and leaves 7 host bits — 2⁷ total addresses.
- In network design terms, /121 is typically a legacy point-to-point size.
- Written out, /121 holds exactly 128 addresses.
- A /121 is 1/2⁵⁷ of a standard /64 LAN subnet.
Design guidance
A /128 is a host route — one specific address. Use it for loopback (::1/128), anycast service endpoints, or static host routes in routing policy. Do not assign a /128 as a LAN prefix.
Practical example
In a lab, 2001:db8:abcd:0012::1/121 might number a small segment with 128 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 /121 blocks, prefix sizes, and use cases.