Dealing with CIDRs in Scheme

I’m always forgetting how to convert a CIDR into a netmask. I remembered the part about (32 - the CIDR) telling you the important part. But I forgot that the important part it tells you is that this gives you the power of 2, and you still need to do some work to get from that to the number of addresses, and the subnet mask you need.

Here is that work!

> (define (cidr slash)
    (let ([power-of-two (- 32 slash)])
      (format "~a addresses, subnet mask: 255.255.255.~a" 
        (expt 2 power-of-two) (- 255 (- (expt 2 power-of-two) 1)))))

> (cidr 26)
"64 addresses, subnet mask: 255.255.255.192"

Now, will this make me better remember how to calculate the subnet mask when I next see a CIDR? Or will I now better forget because I wrote a function to tell me?