More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /5 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 /5
- Prefix mask: f800::
- Total addresses: 2¹²³
- Approx. count: 1.06 × 10³⁷
- /64 subnets: 2⁵⁹
IPv6 /5 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
- Network bits
- 5
- Host bits
- 123
- Prefix mask
- f800::
- Total addresses
- 2¹²³
- Approx. count
- 1.06 × 10³⁷
- /64 subnets
- 2⁵⁹
- Addresses formula
- 2^123
- /64 relationship
- 2⁵⁹ × /64 subnets
Overview
A /5 block fixes the first 5 bits of the address, leaving 123 host bits and 2¹²³ total addresses. It subdivides into about 2⁵⁹ /64 LAN subnets. That is also about 2⁴³ /48 site allocations. At the ISP tier, a /5 holds about 2²⁷ /32 ISP blocks. Blocks this large are used for IANA and RIR-level address planning, not for individual networks.
Common use cases
- IANA and RIR address-space documentation
- Top-level BGP aggregation planning
- IPv6 certification and training exercises
Key facts
- A /5 fixes 5 network bits and leaves 123 host bits — 2¹²³ total addresses.
- In network design terms, /5 is typically a IANA reserved / special-purpose block.
- You can subnet a /5 into about 2⁵⁹ /64 LANs.
- At site scale, /5 equals about 2⁴³ /48 allocations.
- At ISP scale, /5 contains about 2²⁷ /32 blocks.
Design guidance
A /5 is not a size you assign to a LAN or site. Treat it as documentation of how the IPv6 address space is carved at the top of the hierarchy. When studying for certifications, focus on which well-known ranges (2000::/3, fe80::/10, fc00::/7, ff00::/8) live inside or beside this block.
Practical example
Addresses like 2001::/5 fall in reserved or special-purpose space. They illustrate how a /5 boundary groups addresses for routing policy, not how you would number a home LAN.
Related RFCs and standards
- RFC 4291IPv6 Addressing Architecture
- RFC 3849IPv6 Documentation Address Prefix (2001:db8::/32)
Prefix sizing reference
Notable /5 networks
2001:db8::/32Documentation prefix (RFC 3849)
Frequently asked questions
Common questions about IPv6 /5 blocks, prefix sizes, and use cases.