IPv6 Visualizer
Expand it, compress it, and see which part is the prefix and which is the interface identifier — with the compression rules made explicit.
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Groups
Highlighted prefixDimmed interface identifier
Why it compresses that way
The compression rules
IPv6 addresses are 128 bits written as eight groups of four hex digits. Two rules shorten them, and RFC 5952 makes both mandatory for text output so that the same address is always written the same way.
Leading zeros in a group are dropped. 0db8 becomes db8. A group of all zeros becomes 0, never nothing.
One run of zero groups becomes :: — and only one, because two would be ambiguous. The run chosen is the longest; when two runs are the same length, the first wins. A run of a single zero group is never compressed, since :: would save nothing and add ambiguity.
A /64 is the usual subnet size, which is why the interface identifier is shown separately at that length. It is a convention rather than a rule — other lengths are valid and sometimes necessary — but a great deal of IPv6 assumes it.
Related tools
Subnet Calculator handles both families. CIDR Range Calculator shows a prefix's boundaries.