The shift to IPv6 addresses the exhaustion of IPv4’s 32-bit space, with adoption accelerating in 2025 to support IoT and global connectivity, necessitating new subnetting strategies. IPv6 addresses are increasingly replacing IPv4 due to address exhaustion, with adoption surpassing 40% globally by 2025, driving new subnetting needs. We use CIDR, VLSM, and NAT to save as much IPv4 address space as possible. CIDR, VLSM, and NAT are not available in IPv6 addresses. IPv6 subnetting requires a different approach than IPv4 subnetting. Because there are too many addresses, the reason for IPv6 subnetting is different.
The IP version 4 addresses are 32-bit, while the IP version 6 addresses are 128-bit, which allows for more hosts. The IP version 6 allows about 340,282,366,920,938,463,463,374,607,431,768,211,456, or 340 undecillion addresses, almost equal to each particle of sand on the Earth.
The IPv4 subnet limits the broadcast domains, increasing the network’s efficiency and speed. We also required subnetting for better management of IP addresses. The Variable Length Subnet Mask (VLSM) and Fixed Length Subnet Mask help to maintain IPv4 address space. IPv6 subnetting is not concerned with maintaining address space. The /64 is the smallest recommended subnet prefix in IPv6 addresses.
If you have a few devices on your subnet or your network, you must use a /64 prefix address with 264 IP addresses. The critical thing about IPv6 is that it does not use network ID and broadcast addresses. So, an address where the host bits are all 0s or all 1s is still valid. There are two types of assignable IPv6 addresses:
- link-local
- Global Unicast Addresses
Each IPv6-enabled device can create a unique link-local address based on the MAC address of that device using the EUI process.
The IPv6 Global Unicast Addresses
The IP version 6 global unicast address generally consists of a /48 global routing prefix, a 16-bit subnet ID, and a 64-bit interface ID. We already discussed the IPv6 Global Unicast addresses. You can see that the subnet ID is 16 bits, which is more than enough subnets. IPv6 subnetting is about building an addressing hierarchy based on the number of sub-networks needed.
IPv6 Subnetting Using Subnet ID
The 16-bit subnet ID of the IPv6 global unicast address is used to create more subnets. The subnet ID provides enough subnets and hosts support for any organization. The 16-bit section can create 65536/64 subnets without borrowing any bits from the interface ID section of the address. Each subnet supports 18,000,000,000,000,000,000 IPv6 addresses for each subnet. IPv6 subnetting is easier than IPv4 subnetting because there is no binary conversion in IPv6 subnetting. It just required counting in hexadecimal.
Example of Subnetting IPv6 Addresses
Suppose an IP version 6 address 2001:1D11:220A::/48 is assigned to an organization with a 16-bit subnet ID. The network administrators can subnet the IP version 6 address just counting /16-bit subnet ID in hexadecimal. This would allow the administrator to create a 65,536 /64 subnet. The table below illustrates the subnetting procedure of the IPv6 address.
Subnet Table with Usable Addresses
| Subnet ID | Network Address | First Usable Address | Last Usable Address |
|---|---|---|---|
| 0000 | 2001:1D11:220A:0000::/64 | 2001:1D11:220A:0000::1 | 2001:1D11:220A:0000:FFFF:FFFF:FFFF:FFFF |
| 0001 | 2001:1D11:220A:0001::/64 | 2001:1D11:220A:0001::1 | 2001:1D11:220A:0001:FFFF:FFFF:FFFF:FFFF |
| 0002 | 2001:1D11:220A:0002::/64 | 2001:1D11:220A:0002::1 | 2001:1D11:220A:0002:FFFF:FFFF:FFFF:FFFF |
| 0003 | 2001:1D11:220A:0003::/64 | 2001:1D11:220A:0003::1 | 2001:1D11:220A:0003:FFFF:FFFF:FFFF:FFFF |
| 0004 | 2001:1D11:220A:0004::/64 | 2001:1D11:220A:0004::1 | 2001:1D11:220A:0004:FFFF:FFFF:FFFF:FFFF |
| 0005 | 2001:1D11:220A:0005::/64 | 2001:1D11:220A:0005::1 | 2001:1D11:220A:0005:FFFF:FFFF:FFFF:FFFF |
Example of Subnetting IPv6 Addresses
| Subnet ID | Network Address |
|---|---|
| 0000 | 2001:1D11:220A:0000::/48 |
| 0001 | 2001:1D11:220A:0001::/48 |
| 0002 | 2001:1D11:220A:0002::/48 |
| 0003 | 2001:1D11:220A:0003::/48 |
| 0004 | 2001:1D11:220A:0004::/48 |
| 0005 | 2001:1D11:220A:0005::/48 |
| 0006 | 2001:1D11:220A:0006::/48 |
| 0007 | 2001:1D11:220A:0007::/48 |
| 0008 | 2001:1D11:220A:0008::/48 |
| 0009 | 2001:1D11:220A:0009::/48 |
| 000A | 2001:1D11:220A:000A::/48 |
| 000B | 2001:1D11:220A:000B::/48 |
| 000C | 2001:1D11:220A:000C::/48 |
| 000D | 2001:1D11:220A:000D::/48 |
| 000E | 2001:1D11:220A:000E::/48 |
| 000F | 2001:1D11:220A:000F::/48 |
| 0010 | 2001:1D11:220A:0010::/48 |
| 0011 | 2001:1D11:220A:0011::/48 |
| 0012 | 2001:1D11:220A:0012::/48 |
| 0013 | 2001:1D11:220A:0013::/48 |
| 0014 | 2001:1D11:220A:0014::/48 |
| 0015 | 2001:1D11:220A:0015::/48 |
| … | … |
| FFFF | 2001:1D11:220A:FFFF::/48 |
IPv6 Subnet Allocation
An IP version 6 network needs a subnet for each LAN and for the WAN link. WAN links must use /64 subnets to ensure compatibility with autoconfiguration, despite ‘wasting’ addresses, a standard practice as of July 2025. Even if this may “waste” more addresses, that is not a concern when using IP version 6.
As shown in the figure above, there are 6 subnetworks which is allotted six subnets, with the subnet IDs 0000 through 0004 assigned to LANs and a subnet with ID 0005 assigned to WAN links. Every /64 subnet will give more addresses than these Local Area Networks (LANs) require, including Wide Area Networks (WANs). As shown in the Figure, each Local Area Network (LAN) segment and the Wide Area Network (WAN) link are assigned a /64 subnet.
IPv6 Subnetting Best Practices
- Use /64 prefixes for all subnets.
- Leverage 16-bit subnet IDs for hierarchy.
- Verify with show ipv6 interface brief, optimizing CCNA skills as of July 2025.
Troubleshooting IPv6 Subnetting
- Error 1: Duplicate addresses. Fix with clear IPv6 neighbors.
- Error 2: Incorrect prefix. Verify with the show ipv6 route.
Enterprise IPv6 Deployment Case
A company with 2001:1D11:220A::/48 deploys 10 LANs (/64) and 2 WANs (/64), using subnet IDs 0000-0009 and 000A-000B, supporting 10,000 devices with room for growth as of July 1, 2025.
FAQs
What is IPv6 subnetting and why is it important?
IPv6 subnetting divides a large address space into smaller, manageable subnets. It improves network organization, enhances security, and supports efficient routing without the address exhaustion issues of IPv4.
How is IPv6 subnetting different from IPv4?
Unlike IPv4, IPv6 uses 128-bit addresses and does not rely on subnet masks. Instead, it uses prefix lengths (e.g., /64) to define subnets, offering a vastly larger address space and simplified routing.
What prefix length is commonly used in IPv6 subnetting?
A /64 prefix is the standard for most IPv6 subnets, especially those involving end-user devices. It supports stateless address autoconfiguration and ensures compatibility with IPv6 standards.
Can I use VLSM with IPv6?
Yes, IPv6 supports Variable Length Subnet Masking (VLSM) through flexible prefix lengths. This allows network designers to allocate address space based on specific departmental or functional needs.
What tools can help with IPv6 subnetting?
IPv6 subnet calculators and planning tools assist in dividing address blocks, visualizing subnet hierarchies, and ensuring efficient allocation. These tools are essential for large-scale deployments.
