More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /3 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 /3
- Prefix mask: e000::
- Total addresses: 2¹²⁵
- Approx. count: 4.25 × 10³⁷
- /64 subnets: 2⁶¹
IPv6 /3 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
- Network bits
- 3
- Host bits
- 125
- Prefix mask
- e000::
- Total addresses
- 2¹²⁵
- Approx. count
- 4.25 × 10³⁷
- /64 subnets
- 2⁶¹
- Addresses formula
- 2^125
- /64 relationship
- 2⁶¹ × /64 subnets
Overview
The 2000::/3 block is the entire current global unicast address space — every routable IPv6 address handed out today comes from it. A /3 fixes only the first three bits (001), leaving 125 host bits and 2¹²⁵ addresses. The rest of the 128-bit space is reserved by the IETF for future use, link-local, unique-local, and multicast ranges.
Common use cases
- Understanding which addresses are globally routable
- IANA / IETF address-space documentation
- Default-route and global reachability discussions
Key facts
- A /3 fixes 3 network bits and leaves 125 host bits — 2¹²⁵ total addresses.
- In network design terms, /3 is typically a global address-space boundary.
- You can subnet a /3 into about 2⁶¹ /64 LANs.
- At site scale, /3 equals about 2⁴⁵ /48 allocations.
- At ISP scale, /3 contains about 2²⁹ /32 blocks.
Design guidance
A /3 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 2000::/3 fall in reserved or special-purpose space. They illustrate how a /3 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 /3 networks
2000::/3Global unicast space (all public IPv6)::/0Default route — every IPv6 address2001:db8::/32Documentation prefix (RFC 3849)
Frequently asked questions
Common questions about IPv6 /3 blocks, prefix sizes, and use cases.