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!

more examples of different subnets please are you saying
172.18.1.160 and 172.18.1.9 should give 255.255.255.0
i hate 3rd octet calculations

yes, 172.18.1.160 and 172.18.1.9 would be 255.255.255.0

so each of the three calculations would look like 255-abs(ip1-ip2)

I just don't know how to pull the proper numbers without splitting everything into columns

Some additional examples:

172.17.1.200 and 172.18.1.190 would yield 255.254.255.0
172.18.1.100 and 172.18.3.100 would yield 255.255.253.0

try this i think its right

="255."&255-(TRIM(MID(SUBSTITUTE(B1,".",REPT(" ",50)),51,20))-TRIM(MID(SUBSTITUTE(A1,".",REPT(" ",50)),51,20)))&"."&255-(TRIM(MID(SUBSTITUTE(B1,".",REPT(" ",50)),100,30))-TRIM(MID(SUBSTITUTE(A1,".",REPT(" ",50)),100,30)))&".0"

just coz i didnt think you'd have a mask starting less than 255