Counting Unique Strings In A Column

I have a database in which one column contains IP addresses.

Sometimes, an IP address appears more than once in the column.

I need a function that counts the amount of unique IP addresses in that column.

(ie.
With a data set of:

A1: 123.45.67.89
A2: 123.45.67.89
A3: 987.65.43.21

the function would return 2
)

Can anyone enlighten me?