What is IPv6?
IPv6 (Internet Protocol version 6) is the most recent version of the Internet Protocol, designed to replace IPv4 due to address exhaustion. While IPv4 provides approximately 4.3 billion addresses using 32 bits, IPv6 uses 128 bits to provide approximately 340 undecillion (3.4×10³⁸) addresses – enough for every grain of sand on Earth to have its own IP address.
IPv6 addresses are written as eight groups of four hexadecimal digits, separated by colons (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334). This format allows for address compression using specific rules, making addresses more manageable. Our IPv6 subnet calculator helps you work with these addresses for network planning and configuration. For IPv4 networks, use our standard subnet calculator.
IPv6 vs IPv4 Key Difference
IPv6 addresses are 128 bits (vs 32 bits for IPv4), typically displayed in hexadecimal (vs decimal), and use prefix lengths up to /128 (vs /32). The standard subnet for end-user networks is /64, which provides 2⁶⁴ addresses – more than the entire IPv4 address space squared.
How to Use the IPv6 Subnet Calculator
Subnet Calculator Mode
Enter an IPv6 address in any valid format (compressed or expanded) along with the prefix length. The calculator will display the network address, address range, and total number of addresses in the subnet. Common prefix lengths include /48 for site allocations, /56 for customer assignments, and /64 for individual subnets.
Expand/Compress Mode
Enter an IPv6 address to see both its fully expanded form (all 32 hexadecimal characters) and its compressed form (shortest valid representation). This is useful for converting between formats when working with different systems or documentation.
IPv6 Address Format
IPv6 addresses follow specific formatting rules that allow for compression while maintaining uniqueness:
Full Format
The complete format shows all 128 bits as eight 16-bit groups in hexadecimal: 2001:0db8:0000:0000:0000:0000:0000:0001. Each group contains exactly four hexadecimal digits.
Compression Rules
To make addresses more readable, IPv6 allows two compression techniques:
- Leading zeros: Within each group, leading zeros can be omitted.
0db8becomesdb8, and0001becomes1. - Consecutive zero groups: One sequence of consecutive all-zero groups can be replaced with
::. The address2001:0db8:0000:0000:0000:0000:0000:0001becomes2001:db8::1.
Important Rule
The :: shorthand can only be used once per address. Using it twice would make the address ambiguous, as you couldn't determine how many zero groups each :: represents.
Common IPv6 Prefix Lengths
Unlike IPv4 where subnet sizes vary widely, IPv6 has standardized prefix lengths for different allocation purposes:
| Prefix | Addresses | Typical Use |
|---|---|---|
| /32 | 2⁹⁶ (≈7.9×10²⁸) | Regional Internet Registry allocation to ISP |
| /48 | 2⁸⁰ (≈1.2×10²⁴) | Site allocation (organization/enterprise) |
| /56 | 2⁷² (≈4.7×10²¹) | Customer allocation from ISP |
| /64 | 2⁶⁴ (≈1.8×10¹⁹) | Single subnet (standard LAN) |
| /128 | 1 | Single host address (loopback, etc.) |
IPv6 Address Types
IPv6 defines several address types, each serving specific purposes in network communication:
Global Unicast (2000::/3)
These are the equivalent of public IPv4 addresses, routable on the global internet. They begin with binary 001 (hex 2 or 3). The most common range you'll encounter is 2000::/3, which encompasses addresses from 2000:: to 3fff:ffff:ffff:ffff:ffff:ffff:ffff:ffff.
Link-Local (fe80::/10)
Automatically assigned to every IPv6-enabled interface, link-local addresses are used for communication within a single network segment. They're not routable beyond the local link and always begin with fe80. Every device has a link-local address even without DHCP or manual configuration.
Unique Local (fc00::/7)
Similar to IPv4 private addresses (10.x.x.x, 192.168.x.x), unique local addresses are for private networks and are not routed on the public internet. The range fd00::/8 is commonly used for internal networks. These addresses provide a stable addressing scheme for internal communication.
Multicast (ff00::/8)
IPv6 multicast replaces IPv4 broadcast functionality. Addresses beginning with ff are used to send packets to multiple destinations simultaneously. Different multicast scopes (link-local, site-local, global) are defined by the following digits.
Loopback (::1)
The loopback address in IPv6 is simply ::1 (equivalent to 127.0.0.1 in IPv4). It's used for a device to send packets to itself for testing purposes.
IPv6 Subnetting Strategy
IPv6 subnetting differs fundamentally from IPv4 due to the vast address space available:
The /64 Boundary
In IPv6, the /64 prefix is the standard size for a single subnet. The first 64 bits identify the network, and the last 64 bits identify the host (interface identifier). This design supports Stateless Address Autoconfiguration (SLAAC), where devices generate their own interface identifier. Breaking this boundary (using /65 or longer for subnets) can break SLAAC and is generally discouraged.
Hierarchical Allocation
A typical hierarchy for an organization with a /48 allocation:
- /48 - Organization allocation (65,536 possible /64 subnets)
- /56 - Department or building (256 /64 subnets each)
- /64 - Individual VLAN or subnet
Example: Subnetting a /48
Organization receives: 2001:db8:abcd::/48
Available subnets: 2001:db8:abcd:0000::/64 through 2001:db8:abcd:ffff::/64
Total /64 subnets: 65,536
Each /64 subnet contains: 18,446,744,073,709,551,616 addresses
IPv6 and IPv4 Comparison
Understanding the differences helps when transitioning networks or working in dual-stack environments:
| Feature | IPv4 | IPv6 |
|---|---|---|
| Address Length | 32 bits | 128 bits |
| Address Format | Dotted decimal (192.168.1.1) | Hexadecimal with colons (2001:db8::1) |
| Total Addresses | ~4.3 billion | ~340 undecillion |
| Standard Subnet | Varies (/8 to /30) | /64 |
| Broadcast | Yes | No (uses multicast) |
| Auto-configuration | DHCP required | SLAAC or DHCPv6 |
| NAT Required | Common | Not needed |
For IPv4 subnet calculations, use our comprehensive subnet calculator, CIDR calculator, or IP subnet calculator.
IPv6 in Practice
Dual-Stack Networks
Most networks today run both IPv4 and IPv6 simultaneously (dual-stack). Devices have addresses in both protocols and can communicate using either. This transition strategy allows gradual IPv6 adoption while maintaining IPv4 compatibility.
Cloud and Data Centers
Major cloud providers (AWS, Azure, GCP) fully support IPv6. When designing cloud networks, consider enabling IPv6 from the start to avoid retrofitting later. VPCs can be configured with IPv6 CIDR blocks alongside IPv4.
Home Networks
Many ISPs now provide IPv6 connectivity with /56 or /64 allocations to residential customers. Home routers increasingly support IPv6 with prefix delegation, automatically obtaining and distributing IPv6 addresses to internal devices.