Pv4 divides its address space into three classes: A, B, and C. IPv6 does not have a concept of classful networks like IPv4. IPv6 subnets or prefixes can be of arbitrary length without any classful boundaries. An IPv6 address prefix is represented by:
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IPv6 address / Length of prefix
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For example, the following are four different but valid representations of the 64-bit prefix 20010DB80000ABCD: