When working with IPv6 addresses, you'll frequently encounter notation like /64, /56, or /48. These numbers represent prefix lengths in CIDR (Classless Inter-Domain Routing) notation, and they're fundamental to understanding how IPv6 networks are structured and allocated. Unlike IPv4's subnet masks, IPv6 uses prefix lengths to define network boundaries and determine how address space is divided.
This guide explains what these prefix sizes mean, how to calculate the available address space, and which prefix length is appropriate for different networking scenarios.
In CIDR notation, an IPv6 address is followed by a slash and a number (e.g., 2001:db8:abcd::/64). This number indicates the prefix length—how many bits of the 128-bit IPv6 address structure are used to represent the network portion.
Key Concept: The remaining bits (128 minus the prefix length) are available for addressing individual hosts or creating subnets within that network.
Consider the address 2001:db8:cafe:1234::/64:
/64 means the first 64 bits identify the network2001:db8:cafe:1234 (first 64 bits)The prefix length determines how the 128-bit address space is split between network identification and host addressing.
The /64 prefix is the standard and most common subnet size in IPv6. It's not just a convention—it's deeply embedded in IPv6's architectural design.
Architectural Design: RFC 4291 (IPv6 Addressing Architecture) specifies that unicast and anycast addresses are composed of two 64-bit parts:
Stateless Address Autoconfiguration (SLAAC): IPv6 was designed to allow devices to automatically configure their own addresses without DHCP. SLAAC requires a 64-bit interface identifier, making /64 essential for this functionality.
Hardware Optimization: Many routing chips and network devices are optimized for /64 boundaries. Some routers experience performance degradation or resource exhaustion with prefix lengths longer than /64.
RFC Recommendations: RFC 7421 analyzes the 64-bit boundary and confirms that /64 should be the standard for nearly all end-user subnets.
Prefix Length: /64
Network Bits: 64
Host Bits: 64
Total Addresses: 2^64 = 18,446,744,073,709,551,616
This enormous address space means you'll never run out of addresses within a single subnet—a stark contrast to IPv4 where even a /24 (254 usable addresses) can be constraining.
/64 (though link-local addresses are also used for router connections)Best Practice: Always assign a /64 to any network where end-user devices will connect. Never use prefixes longer than /64 for end-user LANs.
A /56 prefix is the recommended minimum allocation for residential customers and small businesses. This provides ample address space for network segmentation without the complexity of managing a larger /48.
Prefix Length: /56
Network Bits: 56
Subnetting Bits: 8
Total /64 Subnets: 2^8 = 256 subnets
Addresses per Subnet: 2^64 ≈ 18.4 quintillion
Total Address Space: 2^72 ≈ 4.7 sextillion addresses
Multiple Subnets: A /56 provides 256 individual /64 subnets, enabling sophisticated dual-stack or home network segmentation:
ISP Standard Practice: Many ISPs worldwide now delegate /56 prefixes to residential customers via DHCPv6 Prefix Delegation (DHCPv6-PD). This automated process assigns the prefix to your home router, which then subdivides it for internal use.
Future-Proof: Even a power user with extensive home networking requirements will rarely need more than 256 subnets.
RIPE Best Practice: RIPE-690 (Best Current Operational Practice) states: "It is strongly discouraged to assign prefixes longer than /56 unless there are very strong and unsolvable technical reasons."
If your ISP assigns you 2001:db8:1234:5600::/56, you can create subnets:
| Subnet ID | Subnet Prefix | Purpose |
|---|---|---|
| 00 | 2001:db8:1234:5600::/64 |
Main LAN |
| 01 | 2001:db8:1234:5601::/64 |
Guest WiFi |
| 02 | 2001:db8:1234:5602::/64 |
IoT Devices |
| 03 | 2001:db8:1234:5603::/64 |
Home Office |
| 04 | 2001:db8:1234:5604::/64 |
Media/Streaming |
| ... | ... | ... |
| FF | 2001:db8:1234:56ff::/64 |
256th subnet |
You have the flexibility to organize these 256 subnets however you need, and each subnet provides trillions of addresses.
Different ISPs use varying approaches:
/48 per customer but initially assign only the first /56, allowing expansion without renumbering/48 to business customers and /56 to residential/60 (16 subnets), though this is discouragedA /48 prefix is the recommended allocation for enterprises, data centers, and larger sites. It's also the minimum prefix size that will be globally routed in BGP (Border Gateway Protocol).
Prefix Length: /48
Network Bits: 48
Subnetting Bits: 16
Total /64 Subnets: 2^16 = 65,536 subnets
Addresses per Subnet: 2^64 ≈ 18.4 quintillion
Total Address Space: 2^80 ≈ 1.2 billion quintillion addresses
Massive Subnet Capacity: 65,536 individual /64 subnets enable hierarchical network design:
Hierarchical Addressing: Large organizations can implement structured addressing schemes:
BGP Routing: A /48 is the minimum prefix size accepted by most ISPs for BGP announcements, making it essential for organizations that need provider-independent addressing.
Industry Standard: RFC recommendations and ARIN/RIPE/APNIC policies typically assign /48 blocks to organizations.
An organization with 2001:db8:1000::/48 could structure it as:
| Purpose | Subnet Range | Number of /64s |
|---|---|---|
| Headquarters (Region 0) | 2001:db8:1000:0000::/52 |
4,096 subnets |
| - Building A | 2001:db8:1000:0000::/56 |
256 subnets |
| - - Floor 1 | 2001:db8:1000:0001::/64 |
1 subnet |
| - - Floor 2 | 2001:db8:1000:0002::/64 |
1 subnet |
| West Coast (Region 1) | 2001:db8:1000:1000::/52 |
4,096 subnets |
| Data Center | 2001:db8:1000:2000::/52 |
4,096 subnets |
| Reserved for Growth | 2001:db8:1000:3000::/52+ |
49,152+ subnets |
This hierarchical design makes routing efficient and network management logical.
| Prefix | Network Bits | Available Bits | Number of /64 Subnets | Typical Use Case |
|---|---|---|---|---|
| /32 | 32 | 96 | 4,294,967,296 | ISP allocation block |
| /40 | 40 | 88 | 16,777,216 | Large ISP, RIR sub-allocation |
| /48 | 48 | 80 | 65,536 | Enterprise, site allocation |
| /52 | 52 | 76 | 4,096 | Large campus, regional subdivision |
| /56 | 56 | 72 | 256 | Residential, small business |
| /60 | 60 | 68 | 16 | Conservative residential (discouraged) |
| /64 | 64 | 64 | 1 | Single subnet/LAN |
| /127 | 127 | 1 | N/A | Point-to-point link (2 addresses) |
| /128 | 128 | 0 | N/A | Single host address |
The formula for calculating address space is straightforward:
Total Addresses = 2^(128 - prefix_length)
Number of /64 Subnets = 2^(64 - prefix_length) [when prefix < 64]
Example 1: How many addresses in a /60?
Total addresses = 2^(128 - 60) = 2^68 = 295,147,905,179,352,825,856
Number of /64 subnets = 2^(64 - 60) = 2^4 = 16 subnets
Example 2: How many addresses in a /56?
Total addresses = 2^(128 - 56) = 2^72 = 4,722,366,482,869,645,213,696
Number of /64 subnets = 2^(64 - 56) = 2^8 = 256 subnets
Example 3: How many addresses in a /48?
Total addresses = 2^(128 - 48) = 2^80 = 1,208,925,819,614,629,174,706,176
Number of /64 subnets = 2^(64 - 48) = 2^16 = 65,536 subnets
The numbers become astronomically large, reinforcing that IPv6's design philosophy is to never worry about address conservation.
Nibble-aligned prefixes (multiples of 4: /48, /52, /56, /60, /64) simplify:
Good: /48, /56, /64
Avoid: /50, /58, /63 (not nibble-aligned)
Never assign prefixes longer than /64 to networks where end-user devices connect. This ensures:
Residential: Request at least a /56 from your ISP. If they offer only /64, you cannot create multiple subnets.
Business: Request a /48 if you anticipate growth or need multiple locations.
Enterprise: Obtain provider-independent space (PI space) from your Regional Internet Registry (RIR) if you need BGP routing.
Before subdividing your allocated prefix:
In IPv4: Conserve addresses, use /30 for point-to-point links, carefully calculate subnet sizes.
In IPv6: Stop worrying about address exhaustion. Focus on clear network hierarchy and logical organization instead.
Allocation: ISP provides 2001:db8:a000::/56
Subnetting Plan:
2001:db8:a000:0000::/64 → Primary LAN (wired)
2001:db8:a000:0001::/64 → WiFi Network
2001:db8:a000:0002::/64 → Guest Network
2001:db8:a000:0010::/64 → IoT Devices (isolated)
2001:db8:a000:0020::/64 → Home Lab
2001:db8:a000:00ff::/64 → Future Use
Each subnet gets 18 quintillion addresses—enough for every device you'll ever own.
Allocation: 2001:db8:b000::/48
Hierarchical Plan:
2001:db8:b000:0000::/56 → Main Office (256 subnets)
└─ 2001:db8:b000:0001::/64 → Office LAN
└─ 2001:db8:b000:0002::/64 → Conference Room
└─ 2001:db8:b000:0010::/64 → Guest WiFi
2001:db8:b000:0100::/56 → Branch Office 1
2001:db8:b000:0200::/56 → Branch Office 2
2001:db8:b000:1000::/56 → Cloud VPN
2001:db8:b000:f000::/56 → Reserved for expansion
This leaves thousands of unused subnets for future growth.
Allocation: RIR assigns 2001:db8:c000::/40 (256 × /48 blocks)
Strategic Allocation:
2001:db8:c000::/48 → North America HQ
2001:db8:c001::/48 → EMEA Office
2001:db8:c002::/48 → APAC Office
2001:db8:c010::/48 → Data Center 1
2001:db8:c011::/48 → Data Center 2
2001:db8:c0f0::/48 → Customer VPNs
Each /48 can be further subdivided as needed, with a consistent addressing policy.
Using /64 as your total allocation: If your ISP only gives you a /64, you cannot create multiple subnets. Push for at least a /56.
Assigning /127 or /126 to point-to-point links unnecessarily: While RFC 6164 allows /127 for efficiency, most use cases are fine with /64.
Treating IPv6 like IPv4: Don't try to conserve addresses. Use generous allocations.
Ignoring nibble boundaries: Non-aligned prefixes make configuration error-prone.
Hardcoding addresses without prefixes: Always configure both the address and prefix length.
Before diving deep into subnetting, ensure your network has proper IPv6 connectivity. Visit test-ipv6.run to check:
Understanding your current IPv6 allocation (if any) helps you plan appropriate subnetting strategies.
Understanding IPv6 prefix notation is essential for modern networking:
/64 is the universal standard for end-user subnets—always use it for LANs/56 provides 256 subnets and is ideal for residential and small business use/48 offers 65,536 subnets and suits enterprises and large sitesBy selecting appropriate prefix sizes and following best practices, you can build scalable, well-organized IPv6 networks that will serve your needs for decades to come. The key mindset shift: stop thinking about address scarcity and start thinking about network design.
Additional Resources: