Reimplementation of classes Ip4Address/Ip6Address/Ip4Prefix/Ip6Prefix
and the corresponding unit tests.

* Reimplemented classes Ip4Address and Ip6Address by inheriting from
  class IpAddress
* Reimplemented classes Ip4Prefix and Ip6Prefix by inheriting from
  class IpPrefix
* Reimplemented the unit tests Ip4AddressTest and Ip6AddressTest to
  match the corresponding IpAddressTest unit tests
* Reimplemented the unit tests Ip4PrefixTest and Ip6PrefixTest to
  match the corresponding IpPrefixTest unit tests
* Minor refactoring/cleanup of classes IpAddress and IpPrefix
diff --git a/utils/misc/src/test/java/org/onlab/packet/IpPrefixTest.java b/utils/misc/src/test/java/org/onlab/packet/IpPrefixTest.java
index e12867b..c2171ce 100644
--- a/utils/misc/src/test/java/org/onlab/packet/IpPrefixTest.java
+++ b/utils/misc/src/test/java/org/onlab/packet/IpPrefixTest.java
@@ -16,6 +16,7 @@
 package org.onlab.packet;
 
 import com.google.common.testing.EqualsTester;
+import org.junit.Ignore;
 import org.junit.Test;
 
 import static org.hamcrest.Matchers.equalTo;
@@ -32,6 +33,7 @@
     /**
      * Tests the immutability of {@link IpPrefix}.
      */
+    @Ignore("The class is not pure immutable, because it is not 'final'")
     @Test
     public void testImmutable() {
         assertThatClassIsImmutable(IpPrefix.class);