What is the first part of an IP address?
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.
IPv6 address
Such an address is generated using two hash functions that take several inputs.
The first uses a public key and a random modifier; the latter being incremented repeatedly until a specific amount of zero bits of the resulting hash is acquired.
The second hash function takes the network prefix and the previous hash value.
The least significant 64 bits of the second hash result is appended to the 64-bit network prefix to form a 128-bit address.