Classless Inter-Domain Routing
An IP address is interpreted as composed of two parts: a network-identifying prefix followed by a host identifier within that network. In the previous classful network architecture, IP address allocations were based on the bit boundaries of the four octets of an IP address. An address was considered to be the combination of an 8, 16, or 24-bit network prefix along with a 24, 16, or 8-bit host identifier respectively. Thus, the smallest allocation and routing block contained only 256 addresses—too small for most enterprises, and the next larger block contained addresses—too large to be used efficiently even by large organizations. This led to inefficiencies in address use as well as inefficiencies in routing, because it required a large number of allocated class-C networks with individual route announcements, being geographically dispersed with little opportunity for route aggregation.
Octet (computing)
An IPv6 address consists of sixteen octets, shown using hexadecimal representation (two digits per octet) and using a colon character (:) after each pair of octets (16 bits also known as hextet) for readability, like this FE80:0000:0000:0000:0123:4567:89AB:CDEF. If two or more consecutive octets equal zero they may be replaced by two following colon characters (::) but this can be used only once in a given IPv6 address to avoid ambiguity. The previously given IPv6 address can thus also be written as FE80::0123:4567:89AB:CDEF. In addition leading zeroes may also be omitted as they are not significant bits in the address. Applying this to the previous example mentioned will result in an IPv6 address of FE80::123:4567:89AB:CDEF.