- What does :: mean in IPv6?
- An abbreviation for one or more zero groups (each group is 16 bits). `2001:db8::1` expands to `2001:0db8:0000:0000:0000:0000:0000:0001`.
- Why expand?
- Comparing addresses programmatically, matching against firewall rules that require exact form, or satisfying tools that don't accept the abbreviated form.
- Can I go the other way?
- Yes — the tool also produces the canonical compressed form (RFC 5952). Same input, whichever direction you need.
- Are leading zeros stripped?
- In compressed form yes. In expanded form, each group has exactly 4 hex digits. Both are valid representations.