your query seems ok and working fine

may be you are selecting LEFT(127.0.0.1,len(127.0.0.1)-charindex('.',reverse(([Ip])))) which may leads you to confusion

you can try putting ip in select and find where's the problem

select LEFT(127.0.0.1,len(127.0.0.1)-charindex('.',reverse(([Ip])))),ip

from @t

group by 127.0.0.1

having count (127.0.0.1)>1

order by (127.0.0.1) desc

or else post some sample data in which you are facing problem.