Hello Srikanth.
Whenever you have two or more groups of four 0’s, you can replace them with ‘::’ So for example, if you have 2001:0db8:85a3:0000:0000:8a2e:0370:7334 it can be rewritten as 2001:0db8:85a3::8a2e:0370:7334.
If however you have only one group of four 0’s such as in 2001:db8:0000:1:1:1:1:1 it is never replaced with a :: but just with a single 0 like this: 2001:db8:0:1:1:1:1:1.
Note also that this two-colon replacement may only be applied once in an address, because multiple occurrences would create an ambiguous representation. For example, 2001:0000:0000:0000:1:0000:0000:1 could be rendered as 2001::1:0:0:1 or 2001:0:0:0:1::1 but not as 2001::1::1
I hope this has been helpful!
Laz