Enhancement: add constants for IPv4 and IPv6 subnet masks

[MatzFan]

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

[taketo1113]

@MatzFan Since IPAddr#prefix and IPAddr#netmask are available, you can convert a netmask to a prefix length and a prefix length to a netmask without defining the proposed constants, as follows:

# IPv4
addr = IPAddr.new("0.0.0.0")
#=> #<IPAddr: IPv4:0.0.0.0/255.255.255.255>

# netmask -> prefix
addr.mask("255.255.0.0").prefix
#=> 16

# prefix -> netmask
addr.mask(16).netmask
#=> "255.255.0.0"


# IPv6
addr = IPAddr.new("::")
#=> #<IPAddr: IPv6:0000:0000:0000:0000:0000:0000:0000:0000/ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff>

# netmask -> prefix
addr.mask("ffff:ffff:ffff:0000:0000:0000:0000:0000").prefix
#=> 48

# prefix -> netmask
addr.mask(48).netmask
#=> "ffff:ffff:ffff:0000:0000:0000:0000:0000"