Netmask to CIDR in ruby

Question

I've been using the ip-address gem and it doesn't seem to have the ability to convert from a netmask of the form

255.255.255.0 

into the CIDR form

/24

Does anyone have an ideas how to quickly convert the former to the latter ?

Answer 1

require 'ipaddr'

def serialize_ipaddr(address)
  mask = address.instance_variable_get(:@mask_addr).to_s(2).count('1')
  "#{address}/#{mask}"
end

serialize_ipaddr(IPAddr.new('192.168.0.1/24')) # => "192.168.0.0/24"

The code achieves the masking by accessing private instance variable *@mask_addr) of IPAddr instance (address, passed into serialize_ipaddr). This is not recommended way (as the instance variables are not part of the classes public API but here it's better than parsing the string from #inspect in my opinion.

So the process is as follows:

1. Get the instance variable @mask_addr that represents netmask
2. Get its binary representation e.g. 255.255.255.0 -> 4294967040 -> 11111111111111111111111100000000
3. Count the 1-s in the base-2 number to get CIDR mask (24)
4. Make up a string consisting the address & mask

EDIT: Added explanation to the implementation as requested by NathanOliver

Answer 2

If you don't need to use ip-address gem, you can do this with the netaddr gem

require 'netaddr'

def to_cidr_mask(dotted_mask)
  NetAddr::CIDR.create('0.0.0.0/'+dotted_mask).netmask
end

to_cidr_mask("255.224.0.0") # => "/11" 

Answer 3

Here's a more mathematical approach, avoiding strings at all costs:

def cidr_mask
    Integer(32-Math.log2((IPAddr.new(mask,Socket::AF_INET).to_i^0xffffffff)+1))
end

with "mask" being a string like 255.255.255.0. You can modify it and change the first argument to just "mask" if "mask" is already an integer representation of an IP address.

So for example, if mask was "255.255.255.0", IPAddr.new(mask,Socket::AF_INET).to_i would become 0xffffff00, which is then xor'd with 0xffffffff, which equals 255.

We add 1 to that to make it a complete range of 256 hosts, then find the log base 2 of 256, which equals 8 (the bits used for the host address), then subtract that 8 from 32, which equals 24 (the bits used for the network address).

We then cast to integer because Math.log2 returns a float.

Answer 4

Here is the quick and dirty way

require 'ipaddr'
puts IPAddr.new("255.255.255.0").to_i.to_s(2).count("1")

There should be proper function for that, I couldn't find that, so I just count "1"

If you're going to be using the function in a number of places and don't mind monkeypatching, this could help:

IPAddr.class_eval
  def to_cidr
    "/" + self.to_i.to_s(2).count("1")
  end
end

Then you get

IPAddr.new('255.255.255.0').to_cidr
# => "/24"

Answer 5

Just as a FYI, and to keep the info easily accessible for those who are searching...

Here's a simple way to convert from CIDR to netmask format:

def cidr_to_netmask(cidr)
  IPAddr.new('255.255.255.255').mask(cidr).to_s
end

For instance:

cidr_to_netmask(24) #=> "255.255.255.0"
cidr_to_netmask(32) #=> "255.255.255.255"
cidr_to_netmask(16) #=> "255.255.0.0"
cidr_to_netmask(22) #=> "255.255.252.0"

Answer 6

Quick and dirty conversion:

"255.255.255.0".split(".").map { |e| e.to_i.to_s(2).rjust(8, "0") }.join.count("1").split(".")

=> I split mask in an Array

.map { |e| e.to_i.to_s(2).rjust(8, "0") }

=> For each element in Array:

.to_i

=> Convert into integer

.to_s(2)

=> Convert integer into binary

.rjust(8, "0")

=> Add padding

=> Map return a Array with same cardinality

.join

=> Convert Array into a full string

.count("1")

=> Count "1" characters => Give CIDR mask

    def mask_2_ciddr mask
      "/" + mask.split(".").map { |e| e.to_i.to_s(2).rjust(8, "0") }.join.count("1").to_s
    end

    mask_2_ciddr "255.255.255.0"
    => "/24"
    mask_2_ciddr "255.255.255.128"
    => "/25"