diff --git a/lib/resolv.rb b/lib/resolv.rb
index 568c879d31..2b81c05a2e 100644
--- a/lib/resolv.rb
+++ b/lib/resolv.rb
@@ -2531,7 +2531,7 @@ class Resolv
     attr_reader :address
 
     def to_s # :nodoc:
-      address = sprintf("%X:%X:%X:%X:%X:%X:%X:%X", *@address.unpack("nnnnnnnn"))
+      address = sprintf("%x:%x:%x:%x:%x:%x:%x:%x", *@address.unpack("nnnnnnnn"))
       unless address.sub!(/(^|:)0(:0)+(:|$)/, '::')
         address.sub!(/(^|:)0(:|$)/, '::')
       end
diff --git a/test/resolv/test_dns.rb b/test/resolv/test_dns.rb
index fe1ab6ce79..d69bec8256 100644
--- a/test/resolv/test_dns.rb
+++ b/test/resolv/test_dns.rb
@@ -216,6 +216,22 @@ class TestResolvDNS < Test::Unit::TestCase
     assert_instance_of Resolv::IPv6, Resolv::IPv6.create('::1:127.0.0.1'), ref
   end
 
+  def test_ipv6_to_s
+    test_cases = [
+      ["2001::abcd:abcd:abcd", "2001::ABcd:abcd:ABCD"],
+      ["2001:db8::1", "2001:db8::0:1"],
+      ["::", "0:0:0:0:0:0:0:0"],
+      ["2001::", "2001::0"],
+      ["2001:db8::1:1:1:1:1", "2001:db8:0:1:1:1:1:1"],
+      ["1::1:0:0:0:1", "1:0:0:1:0:0:0:1"],
+      ["1::1:0:0:1", "1:0:0:0:1:0:0:1"],
+    ]
+
+    test_cases.each do |expected, ipv6|
+      assert_equal expected, Resolv::IPv6.create(ipv6).to_s
+    end
+  end
+
   def test_ipv6_should_be_16
     ref = '[rubygems:1626]'