Most IPv6 addresses do not occupy all of their possible 128 bits. This condition results in fields that are padded with zeros or contain only zeros.

The IPv6 addressing architecture allows you use the two-colon (::) notation
to represent contiguous 16-bit fields of zeros. For example, you might abbreviate
the IPv6 address in Figure 3–2 by
replacing the two contiguous fields of zeros in the interface ID with two
colons. The resulting address is `2001:0db8:3c4d:0015::1a2f:1a2b`.
Other fields of zeros can be represented as a single 0. You can also omit
any leading zeros in a field, such as changing `0db8` to `db8`.

So the address `2001:0db8:3c4d:0015:0000:0000:1a2f:1a2b` can
be abbreviated as `2001:db8:3c4d:15::1a2f:1a2b`.

You can use the two colon notation to replace any contiguous fields
of all zeros in the IPv6 address. For example, the IPv6 address `2001:0db8:3c4d:0015:0000:d234::3eee:0000` can be collapsed into `2001:db8:3c4d:15:0:d234:3eee::`.

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