IPv6 Unique Local addresses (ULAs) are equivalent of private addresses in IPv4 that is, 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These addresses are routable between subnets on a private network but are not routable on the public Internet. They allow you to create complex internal networks without having public address space assigned. ULAs begin with the prefix "fd" and their structure is as follows:
The first seven bits of the address are always 1111 110 (binary) and the eight bit is set to 1 to indicate a local address. Therefore, the address prefix in hex is fd00::/8 for this type of address.
The next 40 bits represent the global ID and is a randomly generated value that identifies a specific site within your organization.
The next 16 bits represent the subnet ID and can be used for further subdividing the internal network of your site for routing purposes.
The last 64 bits are the interface ID and specify a unique interface within each subnet.