IPv6 compression questions are easier to check by expanding the proposed answer back to eight groups. Removing a zero is safe only when the notation rules allow it. Removing the wrong zero changes the address, even if the result looks neater.
Compress this original example
Start with:
2001:0db8:0000:0000:0000:0042:0000:0007
First remove leading zeros within each group:
2001:db8:0:0:0:42:0:7
Then replace the longest consecutive run of all-zero groups with a double colon:
2001:db8::42:0:7
The double colon represents three omitted groups here. There are five explicitly written groups, and a complete IPv6 address has eight. Adding those three zero groups returns you to the original address.
Spot the invalid shortcuts
Consider 2001:db8::42::7. Two double colons make it ambiguous how the missing zero groups should be distributed. Do not use this as an abbreviated form.
Also compare 0042 and 4200. The first can become 42; the second cannot become 42, because those zeros are trailing digits and carry value. IPv6 groups use hexadecimal notation, so treating the address as an ordinary decimal number is another unhelpful shortcut.
Try several variations through the CCNA practice questions for review, then explain why the incorrect alternatives do not expand to the original address. The checking method is more reliable than judging which option appears shortest.
Not every equivalent form is the preferred form
More than one valid textual representation can describe the same address. A question may ask either for a valid abbreviation or for the recommended canonical representation. Read that wording carefully.
RFC 5952 recommends the longest run of zero groups, the leftmost run when equal-length runs tie, and lowercase hexadecimal letters. It also avoids using a double colon to compress just one zero group. Those recommendations help systems produce consistent text without changing the underlying address.
Use a repeatable answer check
Count the explicit groups, calculate how many zero groups the double colon must represent, and restore leading zeros as needed. Confirm that no group exceeds four hexadecimal digits. Only after that should you compare your expanded result with the question.
Keep one correct example and two incorrect examples beside the CCNA cheat sheet. For a ten-minute drill, alternate compression and expansion rather than doing only one direction. A candidate who can expand an answer can check unfamiliar IPv6 questions without depending on the exact address used in a previous practice session.