Worked example: compress 2001:0db8:0000:1111:0000:0000:0000:0200 to its shortest valid form.

2001:db8:0:1111::200 — drop leading zeros, then collapse the longest run of zero hextets (hextets 5-7) with ::.

Step by step:

Start:    2001:0db8:0000:1111:0000:0000:0000:0200
Rule 1:   2001: db8:   0:1111:   0:   0:   0: 200   (drop leading zeros)
        = 2001:db8:0:1111:0:0:0:200
Rule 2:   collapse the longest zero run (the three 0 hextets, positions 5-7)
Result:   2001:db8:0:1111::200

Why not collapse the single 0 in hextet 3?

• :: should replace the longest run; using it on the lone 0 (e.g. 2001:db8::1111:0:0:0:200) is shorter than the original but not the shortest, and :: can be used only once.

Gotcha: 2001:db8:0:1111:0:0:0:200 cannot legally use :: in two places, so you must pick one run — choose the longest.

Go deeper:

• IPv6 address — recommended representation (Wikipedia) — confirms the canonical form collapses the longest zero run (and the leftmost on a tie).