Extend IPAddr with a method to return the value of the network range subnet mask

IPAddrWithMask.rb

require 'ipaddr'

class IPAddrWithMask < IPAddr
    def mask()
      ## use _to_string private method from IPAddr class
      _to_string(@mask_addr)
    end ### def
end ### class

net  = IPAddrWithMask.new("172.23.0.0/24")

puts net.mask
### 255.255.255.0