IPv6 private addressing should be very limited compared to its use in IPv4. IPv6 private IP addresses are referred to as unique local addresses (ULAs) and use the prefix FC00::/7. In both small and large companies, you should not expect to use ULAs in IPv6 networks. Furthermore, the Internet Engineering Task Force (IETF) does not recommend the use of NAT for IPv6. In the remote event that ULAs are needed, you will also use NAT66 for the IPv6-to-IPv6 private-to-public translation.
IPv6 for the Enterprise
IPv6 addresses are assigned in a hierarchical manner. IANA allocates IPv6 addresses to the RIRs. The RIRs, in turn, allocate address blocks to Local Internet Registries (LIRs), and most LIRs are ISPs. In some regions (for example, APNIC), RIRs allocate addresses to National Internet Registries (NIRs), which in turn allocate addresses to ISPs. Normally, ISPs are allocated /32 blocks of addresses. Companies are allocated address blocks from /40 to /64. Large companies are allocated a /40 block of IPv6 addresses. Small companies might receive a /56 block, but a normal allocation is a /48 block of IPv6 addresses. Private consumers, such as residential users, are allocated a /64 address block.
A /48 address block is equal to 216 (that is, 65,536) /64 subnets. As an example, if a company is allocated 2001:DB8:0ABC::/48, this allows the company to assign subnets from 2001:DB8:0ABC:0000::/64, 2001:DB8:0ABC:0001::/64, 2001:DB8:0ABC:0002::/64, all the way to 2001:DB8:0ABC:FFFF::/64. That is 216 = 65,536 subnets!
IPv6 Address Allocation
There are several schemas to allocate IPv6 addresses within an organization. Because IPv6 addresses are usually allocated to a network that already has IPv4 addresses, you can attempt to use the IPv4 address or subnet as part of the IPv6 address. You can also allocate IPv6 address space to show a location and type.
Partly Linked IPv4 Address into IPv6
IPv6 deployments are not expected to be greenfield; there will be IPv4 subnets on the network. One method to allocate IPv6 addresses is to match the IPv6 /64 subnet with the IPv4 /24 subnet. In addition, the IP subnet can match the VLAN number used. Table 2-10 shows an example. The third octet of the IPv4 subnet is used as the subnet for the IPv6 /64 subnet; furthermore, it matches the VLAN number. Note that this works very well with IPv4 /24 subnets, but it does not work optimally with /30 and other smaller links.
Table 2-10 IPv6 Address Allocation Partly Linked to IPv4
| VLAN Number | IPv4 Subnet | IPv6 Subnet |
| VLAN 11 | 172.16.11.0/24 | 2001:DB8:ABC:11::/64 |
| VLAN 12 | 172.16.12.0/24 | 2001:DB8:ABC:12::/64 |
| VLAN 13 | 172.16.13.0/24 | 2001:DB8:ABC:13::/64 |
| VLAN 111 | 172.16.111.0/24 | 2001:DB8:ABC:111::/64 |
| VLAN 112 | 172.16.112.0/24 | 2001:DB8:ABC:112::/64 |
| VLAN 113 | 172.16.113.0/24 | 2001:DB8:ABC:113::64 |