There are a ton of ways to do this but they all usually depend on certain things being there unless you happen to be pretty good at writing AI code. 😛

Assuming that the only periods in the whole string will be a part of the IP address and the IP address will always be immediately followed by a colon, this is one of the simplest methods.

--===== Setup the test. This is NOT a part of the solution.

DECLARE @SomeString VARCHAR(8000);

SELECT @SomeString = 'jdbc:inetpool:inetdae:10.25.81.123:17100?database=us_nnnn';

--===== Demonstrate the solution which could be modified to work on a table column.

SELECT SUBSTRING(PARSENAME(@SomeString,1),1,CHARINDEX(':',PARSENAME(@SomeString,1))-1);