More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /20 subnets
- Expanded address
- Compressed address
- Network (expanded)
- Last address (expanded)
- Prefix mask
- Total addresses (exact)
- Reverse DNS (PTR)
- Host bits / network bits
Hextet breakdown
Quick facts for IPv6 /20
- Prefix mask: ffff:f000::
- Total addresses: 2¹⁰⁸
- Approx. count: 3.25 × 10³²
- /64 subnets: 2⁴⁴
IPv6 /20 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
- Network bits
- 20
- Host bits
- 108
- Prefix mask
- ffff:f000::
- Total addresses
- 2¹⁰⁸
- Approx. count
- 3.25 × 10³²
- /64 subnets
- 2⁴⁴
- Addresses formula
- 2^108
- /64 relationship
- 2⁴⁴ × /64 subnets
Overview
A /20 is a very large aggregation block — bigger than a /32 ISP allocation but smaller than an RIR /12. It contains 2¹⁰⁸ addresses and can be divided into 4,096 /32 ISP blocks or over 2⁴⁴ standard /64 LANs. You will rarely see a /20 assigned to a single organization; it is mainly used in routing tables and address-space documentation.
Common use cases
- Aggregating many /32 ISP allocations under one route
- Top-tier provider backbone planning
- Address-space documentation and training
Key facts
- A /20 fixes 20 network bits and leaves 108 host bits — 2¹⁰⁸ total addresses.
- In network design terms, /20 is typically a RIR-level aggregation block.
- You can subnet a /20 into about 2⁴⁴ /64 LANs.
- At site scale, /20 equals about 2²⁸ /48 allocations.
- At ISP scale, /20 contains 2¹² /32 blocks.
Design guidance
A /20 belongs in BGP aggregation and RIR allocation planning — not on a VLAN interface. If you are subnetting for a real deployment, work downward: carve /48 or /56 site blocks first, then assign one /64 per LAN. Announcing a single aggregated /20 (or larger) upstream keeps global routing tables manageable.
Practical example
An ISP holding 2001:db8::/20 might announce the entire /20 to upstream providers as one BGP route, then delegate /48 blocks such as 2001:db8:0001::/48 and 2001:db8:0002::/48 to business customers. Each customer subnets their /48 into /64 LANs.
Related RFCs and standards
- RFC 4291IPv6 Addressing Architecture
- RFC 3849IPv6 Documentation Address Prefix (2001:db8::/32)
Prefix sizing reference
Notable /20 networks
2001:db8::/32Documentation prefix (RFC 3849)
Frequently asked questions
Common questions about IPv6 /20 blocks, prefix sizes, and use cases.