One of the two default address blocks that are part of every Address Manager configuration is the Unique Local Address Space block. The Unique Local Address Space is similar to the RFC 1918 private address space blocks (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16).
Unique local addresses are useful for sites that aren't always connected to the Internet, or for sites that you want to have a distinct prefix to localize traffic internally. Routers with interfaces on a local network should be configured so that these addresses don't reach the Internet.
Similar to the way in which an IPv4 address is divided into a network ID and a host ID (and sometimes a subnet ID), an IPv6 address is divided into several sections: a prefix, a global ID, a subnet ID and an interface ID. The following table is adapted from RFC 4193.
| Prefix | L | Global ID | Subnet ID | Interface ID |
| 7 bits | 1 bit | 40 bits | 16 bits | 64 bits |
The first seven bits of a local IPv6 address (considered to be the prefix) are set to 1111110. The eighth bit is set to 1. Taken together this provides for an octet of 11111101, or FD in hex. This means that every IP address that's part of the Unique Local Address space begins with FD. The first 8 bits in hexadecimal form are FD00::/8.
The next 40 bits of the address are known as the Global ID. This part of the address is generated by the administrator and it should be unique. Address Manager uses a randomization engine based on RFC 4193 to ensure uniqueness. The randomization is based on the Address Manager server’s MAC address. An example of the prefix and global ID is FD65:80F6:7DFE::/48.
The next 16 bits of the local address are known as the subnet ID. This portion can be subnetted into different sized blocks by the administrator, depending on the network requirements. The subnet ID generally consists of a series of blocks and networks.
The final 64 bits are known as the interface ID. This part is assigned to a host’s interface.