The most significant change from IPv4 to IPv6 is the length of the network address. IPv6 addresses defined by RFC 2373 and RFC 2374 are 128 bits long, as described below; the representation of an IPv6 address generally uses 32 hexadecimal digits.
There are approximately 3.4×10^38 possible addresses in IPv6. It can also be imagined as 2^128, as each bit in a 128-bit address can take on 16 different values (refer to combinatorial mathematics).
In many cases, IPv6 addresses are logically divided into two parts: a 64-bit network prefix and a 64-bit host address. The host address is usually automatically generated based on the physical address, known as EUI-64 (or 64-bit Extended Unique Identifier).
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