site-local address and its definition

http://www.cisco.com/en/US/products/...d800c1136.html
site-local address - Address that is useful only in the context of the site and is similar to the private addresses in IPv4. Its scope is limited to this context. When configured, a site-local address uses a specific prefix (FEC0::/10) and concatenates the subnet ID as a 16-bit field and then the interface ID in the modified EUI-64 format. See also anycast address, global unicast address, IPv6 multicast address, link-local address, and solicited-node multicast address.

How can I manage to force the hex decimal "FEC0" within the limit of 10 bits ?