There must be at least one digit in every group of four digits. That is the reason, the groups with all four zeros in the mentioned address are replaced with a single zero.//
In order to make writing IPv6 addresses with long strings of zero bits easier, a method is available to compress zeros. The special symbol :: can be used to represent one or more adjacent groups of 16 bits of zeros. The symbol :: can appear only once in an address though. The :: can also include leading zeros after the last group of 16 zero bits, or before the first group of 16 zero bits. For example, the same address we mentioned can be squeezed further as: