Working with multiple delimiters in a string

I'm working on creating an automatically generated procedure for computer setup, and where I'm at right now is automatically generating a subnet mask based on IP address and gateway.

So, I have two strings I need to compare the first three sets of numbers of, e.g. 172.18.1.160 and 172.18.0.9 which should generate 255.255.254.0

The actual comparison and generation of the subnet mask is the simple part - I just want to do the comparison without splitting everything into more columns, and limit it to the cell the subnet will be output into if possible.

Thanks for any help, it's greatly appreciated!