diff --git a/lib/puppet/functions/extlib/netmask_to_cidr.rb b/lib/puppet/functions/extlib/netmask_to_cidr.rb
new file mode 100644
index 0000000..13f4990
--- /dev/null
+++ b/lib/puppet/functions/extlib/netmask_to_cidr.rb
@@ -0,0 +1,18 @@
+# @summary Converts an octet netmask address of the form 255.255.255.0 into its CIDR variant.
+#          Thus making it directly usable with the values from facter.
+#
+Puppet::Functions.create_function(:'extlib::netmask_to_cidr') do
+  # @param netmask IPv6 or IPv4 netmask in octet notation
+  # @return CIDR / prefix length
+  # @example calling the function
+  #   extlib::netmask_to_cidr('255.0.0.0')
+  dispatch :netmask_to_cidr do
+    param 'Stdlib::IP::Address::Nosubnet', :netmask
+    return_type 'Integer[0, 128]'
+  end
+
+  def netmask_to_cidr(netmask)
+    dummy_addr = IPAddr.new(netmask).ipv6? ? '::' : '0.0.0.0'
+    IPAddr.new("#{dummy_addr}/#{netmask}").prefix
+  end
+end
diff --git a/spec/functions/extlib/netmask_to_cidr.rb b/spec/functions/extlib/netmask_to_cidr.rb
new file mode 100644
index 0000000..5b0c02e
--- /dev/null
+++ b/spec/functions/extlib/netmask_to_cidr.rb
@@ -0,0 +1,35 @@
+require 'spec_helper'
+
+describe 'extlib::netmask_to_cidr' do
+  it 'exists' do
+    is_expected.not_to be_nil
+  end
+
+  context 'when called with no parameters' do
+    it { is_expected.to run.with_params.and_raise_error(ArgumentError) }
+  end
+
+  context 'when called with a Integer' do
+    it { is_expected.to run.with_params(42).and_raise_error(ArgumentError) }
+  end
+
+  context 'when called with a String thats not an ip address' do
+    it { is_expected.to run.with_params('42').and_raise_error(ArgumentError) }
+  end
+
+  context 'when called with an IP Address that is not in the pure octet notation' do
+    it { is_expected.to run.with_params('127.0.0.1/8').and_raise_error(ArgumentError) }
+  end
+
+  context 'when called with an IP Address that is not in the pure octet notation' do
+    it { is_expected.to run.with_params('fe80::800:27ff:fe00:0').and_raise_error(ArgumentError) }
+  end
+
+  context 'when called with an IPv4 octet netmask' do
+    it { is_expected.to run.with_params('255.0.0.0').and_return(8) }
+  end
+
+  context 'when called with an IPv6 octet netmask' do
+    it { is_expected.to run.with_params('ffff:ffff::').and_return(32) }
+  end
+end