gistfile1.txt

require 'netaddr' ##Version Puppet 3.8.7 

module Puppet::Parser::Functions
  newfunction(:ip_2_network, :type => :rvalue) do |args|
  ## receive the 192.168.10.100/24 ip adderss
    definition = args[1]
    cidr = NetAddr::CIDR.create(args[0])
    if definition == 'ip'
       result = cidr.ip
    elsif definition == 'network'
       result = cidr.network
    elsif definition == 'cidr'
       result = cidr.to_s
    end
    result
  end
end

Using in manisfest as:
$var = ip_2_network("192.168.10.100/24", "ip")