Adding subnet method for Ipv6 objects

lib/ipaddress/ipv6.rb

@@ -549,6 +549,38 @@ def network
       self.class.parse_u128(network_u128, @prefix)
     end
 
+    #
+    # This method implements the subnetting function 
+    # similar to the one described in RFC3531.
+    #
+    # By specifying a new prefix, the method calculates
+    # the network number for the given IPv6 object
+    # and calculates the subnets associated to the new
+    # prefix.
+    #
+    # For example, given the following network:
+    #
+    #   ip = IPAddress "2001:db8:8:800::/64"
+    #
+    # we can calculate the subnets with a /66 prefix
+    #
+    #   ip.subnets(66).map{&:to_string)
+    #     #=> ["2001:db8:8:800::/66", "2001:db8:8:800:4000::/66", "2001:db8:8:800:8000::/66", 
+    #      "2001:db8:8:800:c000::/66"]
+    #
+    # The resulting number of subnets will of course always be
+    # a power of two.
+    #
+
+    def subnet(subprefix)
+      unless ((@prefix.to_i)..128).include? subprefix
+        raise ArgumentError, "New prefix must be between #@prefix and 128"
+      end
+      Array.new(2**([email protected]_i)) do |i|
+        self.class.parse_u128(network_u128+(i*(2**(128-subprefix))), subprefix)
+      end
+    end
+
     #
     # Extract 16 bits groups from a string
     #

test/ipaddress/ipv6_test.rb

@@ -141,6 +141,18 @@ def test_method_size
     ip = @klass.new("2001:db8::8:800:200c:417a/124")
     assert_equal 2**4, ip.size
   end
+####Fill out proper ipv6 values... not sure how to do that math by hand yet.
+  def test_method_subnet
+    assert_raises(ArgumentError) {@network.subnet(62)}
+    assert_raises(ArgumentError) {@network.subnet(129)}
+    arr = ["2001:db8:8:800::/66", "2001:db8:8:800:4000::/66", "2001:db8:8:800:8000::/66", 
+           "2001:db8:8:800:c000::/66"]
+    assert_equal arr, @network.subnet(66).map {|s| s.to_string}
+    arr = ["2001:db8:8:800::/65", "2001:db8:8:800:8000::/65"]
+    assert_equal arr, @network.subnet(65).map {|s| s.to_string}
+    arr = ["2001:db8:8:800::/64"]
+    assert_equal arr, @network.subnet(64).map {|s| s.to_string}
+  end
 
   def test_method_include?
     assert_equal true, @ip.include?(@ip)