IPv6 subnetting standards
IPv6 prefix length cheat sheet.
Unlike IPv4, IPv6 subnetting is designed around hierarchy and nibble boundaries rather than host count conservation. Here is the complete engineering guide to IPv6 prefix sizes.
Standard Prefix Allocations
| Prefix | Possible /64 Subnets | Designated Role | Architectural Application | Nibble Aligned? |
|---|---|---|---|---|
| /32 | 4,294,967,296 | ISP / LIR Allocation | Allocated by Regional Internet Registries (RIRs like ARIN, RIPE) to Internet Service Providers. | Yes (Hex Char) |
| /48 | 65,536 | Enterprise End-Site | Standard allocation from ISP to enterprise, campus, or multi-site organization. | Yes (Hex Char) |
| /52 | 4,096 | Medium Organization | Common delegation for mid-size branch offices or multi-department companies. | Yes (Hex Char) |
| /56 | 256 | Small Business / Residential | Standard ISP delegation for small business and advanced residential broadband connections. | Yes (Hex Char) |
| /60 | 16 | Small Home Office | Minimal ISP residential delegation allowing up to 16 distinct local VLANs. | Yes (Hex Char) |
| /64 | 1 | Standard End-User LAN Subnet | The single universal subnet size for all client LANs and Wi-Fi networks. Required for SLAAC (RFC 4862). | Yes (Hex Char) |
| /126 | N/A | Legacy Point-to-Point | Older router-to-router link allocation providing 4 addresses. | Bit Subnet |
| /127 | N/A | Modern Point-to-Point (RFC 6164) | Recommended for inter-router links. Provides exactly 2 addresses, preventing ping-pong routing loops. | Bit Subnet |
| /128 | N/A | Host / Loopback | Single host interface address or router Loopback0 management interface. | Yes (Hex Char) |
IPv6 Address Scopes & Types
| Prefix Range | Address Type | Scope | Description |
|---|---|---|---|
| 2000::/3 | Global Unicast (GUA) | Public Internet | Globally routable public addresses (similar to public IPv4). |
| fc00::/7 | Unique Local (ULA) | Private Enterprise | RFC 4193 private addresses (equivalent to RFC 1918 private IPv4). Non-routable on the public internet. |
| fe80::/10 | Link-Local Unicast | Single L2 Segment | RFC 4291 automatically assigned to every IPv6 interface. Only valid within the local broadcast domain. |
| ff00::/8 | Multicast | Variable | Replaces broadcast in IPv6. Delivers packets to all nodes subscribing to a multicast group. |
| ::1/128 | Loopback | Host Internal | Equivalent to 127.0.0.1 in IPv4 (localhost). |
| 2001:db8::/32 | Documentation | RFC 3849 | Reserved exclusively for books, documentation, and training examples. |
Understanding Nibble Boundaries in IPv6
In IPv6, each hexadecimal character represents 4 binary bits (1 nibble). Because each group between colons is 16 bits (4 hex characters), subnetting on boundaries that are multiples of 4 bits (/48, /52, /56, /60, /64) ensures each subnet directly aligns with a character in the written address.
Example: Delegated /48 Prefix 2001:0db8:abcd::/48
Bits 0-47 (Global Routing): 2001:0db8:abcd
Bits 48-63 (Subnet ID, 4 hex chars = 65,536 subnets): XXXX
Bits 64-127 (Interface ID / Host Portion): YYYY:YYYY:YYYY:YYYY
This design makes routing tables intuitive to parse visually and eliminates the need to calculate complex bitwise subnet masks manually.
Related tools and guides
- IPv6 Subnet CalculatorPrefix ranges, /64 counts and compressed addresses.
- IP Subnet CalculatorIPv4 network, broadcast and host range from any IP and mask.
- CIDR CalculatorConvert CIDR notation to masks, ranges and host counts.
- Subnetting ExplainedSubnetting vs supernetting, with worked examples.
- Subnet Cheat SheetEvery CIDR prefix with mask, wildcard and host count.
- IP Address ClassesClass A–E ranges, default masks and private IP ranges.