Thanks Dwain.

I tried

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

from [dbo].[TestTable]

but am getting

Incorrect syntax near the keyword 'from'.

127.0.0.1 is column name.

Since I'll probably have a list of over 100k I don't think the other options would be practical.