It’s a really complicated rule called “subtraction”. Addresses are always 128 bits long, or 8 groups of four hex digits. 2000::1 is two groups, so you need six groups in between to make 2000:0000:0000:0000:0000:0000:0000:1. But I don’t know why people always ask this, because it’s always the computer you are typing addresses in to that does the subtraction. You never ever have to type out the whole address. Just type the shortened version, because 2000::1 _is_ the whole address.

They were answering the question of why "2000::1::1" would be ambiguous if it was allowed.