Worked example: expand the compressed address 2001:db8:0:1111::200 to its full 8-hextet form.

2001:0db8:0000:1111:0000:0000:0000:0200 — pad each hextet to 4 digits and replace :: with three 0000 hextets.

Step by step:

Compressed:  2001:db8:0:1111::200
Hextets present: 2001, db8, 0, 1111, 200  = 5 hextets
Missing:     8 - 5 = 3 zero hextets, located where the :: is
Pad + insert:
Result:      2001:0db8:0000:1111:0000:0000:0000:0200

Check: counting the result gives 8 hextets — 2001 | 0db8 | 0000 | 1111 | 0000 | 0000 | 0000 | 0200. The :: sat between 1111 and 200, so the three inserted 0000 hextets go there.

Go deeper:

• IPv6 address (Wikipedia) — the representation rules in reverse: pad each hextet to 4 digits, then expand :: to (8 − present) zero hextets.