Just a simple CIDR to netmask conversion

cidr_to_netmask.rb

def unpack_ip(packed_ip)
  octets = []
  4.times do
    octet = packed_ip & 0xFF
    octets.unshift(octet.to_s)
    packed_ip = packed_ip >> 8
  end
  ip = octets.join('.')
end

def cidr_to_packed_ip(cidr)
  (("1"*cidr)+("0"*(32-cidr))).to_i(2)
end

def cidr_to_netmask(cidr)
  unpack_ip(cidr_to_packed_ip(Integer(cidr)))
end

require 'ipaddr'
def network_addr(ip_addr, cidr)
  packed_ip   = IPAddr.new(ip_addr).to_i
  packed_cidr = cidr_to_packed_ip(cidr)
  unpack_ip(packed_cidr & packed_ip)
end

describe "#cidr_to_netmask" do
  [
    [32, "255.255.255.255"],
    [27, "255.255.255.224"],
    [24, "255.255.255.0"],
    [17, "255.255.128.0"],
    [16, "255.255.0.0"],
    [ 8, "255.0.0.0"],
  ].each do |cidr, netmask|
    specify "for #{cidr} should generate #{netmask}" do
      cidr_to_netmask(cidr).should == netmask
    end
  end
end

describe "#network_addr" do
  [
    ["192.168.0.0",   32, "192.168.0.0"],
    ["192.168.0.10",  24, "192.168.0.0"],
    ["64.131.93.176", 27, "64.131.93.160"],
  ].each do |ip, cidr, network|
    specify "for #{ip}/#{cidr} should generate #{network}" do
      network_addr(ip, cidr).should == network
    end
  end
end