Enhancement: add constants for IPv4 and IPv6 subnet masks

Rationale: This SO question demonstrates there is at least some need to manipulate netmasks into CIDR integers. The accepted answer contains a bug which can be easily eliminated by hard coding the 2 sets of netmasks into the IPAddr class - see my alternative answer.

Proposed solution: Define IPAddr::IPV4_SUBNET_MASKS and IPAddr::IPV6_SUBNET_MASKS as follows:

class IPAddr
  IPV4_SUBNET_MASKS = (0..32).map { |n| IPAddr.new("0.0.0.0/#{n}").netmask }
  IPV6_SUBNET_MASKS = (0..128).map { |n| IPAddr.new("0::/#{n}").netmask }
end

# Now a user can do the following to get a subnet mask from a CIDR integer:
IPAddr::IPV4_SUBNET_MASKS[24]
# => "255.255.255.0"

IPAddr::IPV6_SUBNET_MASKS[64]
# => "ffff:ffff:ffff:ffff:0000:0000:0000:0000"

Optional: Add a predicate to check if an instance of IPAddr is a netmask:

class IPAddr
  def netmask?
    if ipv4?
      IPV4_SUBNET_MASKS.include? to_s
    else
      IPV6_SUBNET_MASKS.include? to_s
    end
  end
end

ipaddr1 = IPAddr.new '255.254.255.0'
ipaddr1.netmask? # => false

ipaddr2 = IPAddr.new '255.255.255.0/24'
ipaddr2.netmask? # => true

ipaddr3 = IPAddr.new '255.255.255.0'
ipaddr3.netmask? # => true

# And so on for IPv6 addresses

Commencez dans la classe IPAddr et examinez la gestion existante d’IPv4, d’IPv6 et des masques réseau. Ajoutez une couverture pour les constantes de masque de sous-réseau proposées et, si le prédicat facultatif est inclus, pour ses cas IPv4 et IPv6 également ; le travail est terminé lorsque les index CIDR renvoient les masques documentés et que les masques non contigus sont rejetés.