The 16-bit subnet ID section of the IPv6 global unicast address can be used by an organization to create internal subnets.
The subnet ID provides more than enough subnets and host support than will ever be needed in one subnet. For instance, the 16-bit section can:
Create up to 65,536 /64 subnets. This does not include the possibility of borrowing any bits from the interface ID of the address.
Support up to 18 quintillion host IPv6 addresses per subnet (i.e., 18,000,000,000,000,000,000).
Note: Subnetting into the 64-bit Interface ID (or host portion) is also possible but it is rarely required.
IPv6 subnetting is also easier to implement than IPv4, because there is no conversion to binary required.To determine the next available subnet, just count up in hexadecimal.
For example, assume an organization has been assigned the 2001:0DB8:ACAD::/48 global routing prefix with a 16 bit subnet ID. This would allow the organization to create /64 subnets.
Notice how the global routing prefix is the same for all subnets. Only the subnet ID hextet is incremented in hexadecimal for each subnet.
As shown in Figure 1, the example topology will require subnets for each LAN as well as for the WAN link between R1 and R2. Unlike the example for IPv4, with IPv6 the WAN link subnet will not be subnetted further. Although this may “waste” addresses, that is not a concern when using IPv6.
As shown in Figure 1, the allocation of five IPv6 subnets, with the subnet ID field 0001 through 0005 will be used for this example. Each /64 subnet will provide more addresses than will ever be needed.
As shown in Figure 1, each LAN segment and the WAN link is assigned a /64 subnet.
Similar to configuring IPv4, Figure 1 shows that each of the router interfaces has been configured to be on a different IPv6 subnet.