function to detect if an IPv4 address should be considered local

is_internal_ip.rb

def internal_ip?(s)
  s =~ /^(\d+)\.(\d+)\.(\d+)\.(\d+)$/
  # based on http://www.ietf.org/rfc/rfc3330.txt
  return case $1 
  when '0', '10', '127'
    true
  
  when '169'
    # link local                     Zero Conf
    (('16'..'31').include? $2) || ($2.eql? '254')

  when '172'
    ('16'..'31').include? $2 
   
  when '192'
    $2.eql? '168'
    
  when ('224'..'239'), ('240'..'255')
    true

  else
    false
  end

end

you can test it like so:

ips = ['127.0.0.2', '192.118.0.33', '192.168.3.33', '10.0.3.45',
'0.0.0.0', '169.19.3.1', '172.16.9.1',
'255.255.255.255', '225.2.2.1', '169.254.0.57']

ips.each do |i|
puts "#{i}=#{internal_ip?(i)}"
end