#!/usr/bin/env ruby

def ip2num(ip)
  ipa = ip.split(".").map(&:to_i)
  return (ipa[0] * (256 ** 3)) + (ipa[1] * (256 ** 2)) + (ipa[2] * 256) + ipa[3]
end

def num2ip(ipi)
  outa = []
  outa[3] = ipi % 256
  outa[2] = (ipi / 256) % 256
  outa[1] = (ipi / (256 ** 2)) % 256
  outa[0] = (ipi / (256 ** 3)) % 256
  return outa.map(&:to_s).join(".")
end

def boundarycheck(ip, cidr)
  bound = 2**(32-cidr)
  return (ip2num(ip) % bound) == 0
end

def addip(ip, x)
  ipi = ip2num(ip)
  ipi += x
  return num2ip(ipi)
end

def reverseip(ip)
  return ip.split('.').reverse.join('.')
end

if ARGV.length == 0 then
  puts "Usage: genrdns ip[/cidr]"
  exit
end

IPCIDR = ARGV.shift.split("/")
IP = IPCIDR[0]
CIDR = IPCIDR[1].nil? ? 32 : IPCIDR[1].to_i

unless boundarycheck(IP, CIDR) then
  puts "Invalid CIDR Boundary"
  exit 1
end

(2**(32-CIDR)).times do |x|
  puts "#{reverseip(addip(IP, x))}.in-addr.arpa"
end