diff --git a/core/java/android/net/NetworkUtils.java b/core/java/android/net/NetworkUtils.java
index 8003afb..4f35a22 100644
--- a/core/java/android/net/NetworkUtils.java
+++ b/core/java/android/net/NetworkUtils.java
@@ -230,6 +230,25 @@
     }
 
     /**
+     * Convert an IPv4 netmask to a prefix length, checking that the netmask is contiguous.
+     * @param netmask as a {@code Inet4Address}.
+     * @return the network prefix length
+     * @throws IllegalArgumentException the specified netmask was not contiguous.
+     * @hide
+     */
+    public static int netmaskToPrefixLength(Inet4Address netmask) {
+        // inetAddressToInt returns an int in *network* byte order.
+        int i = Integer.reverseBytes(inetAddressToInt(netmask));
+        int prefixLength = Integer.bitCount(i);
+        int trailingZeros = Integer.numberOfTrailingZeros(i);
+        if (trailingZeros != 32 - prefixLength) {
+            throw new IllegalArgumentException("Non-contiguous netmask: " + Integer.toHexString(i));
+        }
+        return prefixLength;
+    }
+
+
+    /**
      * Create an InetAddress from a string where the string must be a standard
      * representation of a V4 or V6 address.  Avoids doing a DNS lookup on failure
      * but it will throw an IllegalArgumentException in that case.
@@ -309,6 +328,22 @@
     }
 
     /**
+     * Returns the implicit netmask of an IPv4 address, as was the custom before 1993.
+     */
+    public static int getImplicitNetmask(Inet4Address address) {
+        int firstByte = address.getAddress()[0] & 0xff;  // Convert to an unsigned value.
+        if (firstByte < 128) {
+            return 8;
+        } else if (firstByte < 192) {
+            return 16;
+        } else if (firstByte < 224) {
+            return 24;
+        } else {
+            return 32;  // Will likely not end well for other reasons.
+        }
+    }
+
+    /**
      * Utility method to parse strings such as "192.0.2.5/24" or "2001:db8::cafe:d00d/64".
      * @hide
      */
diff --git a/core/tests/coretests/src/android/net/NetworkUtilsTest.java b/core/tests/coretests/src/android/net/NetworkUtilsTest.java
new file mode 100644
index 0000000..8d51c3b
--- /dev/null
+++ b/core/tests/coretests/src/android/net/NetworkUtilsTest.java
@@ -0,0 +1,70 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.net;
+
+import android.net.NetworkUtils;
+import android.test.suitebuilder.annotation.SmallTest;
+
+import java.net.Inet4Address;
+import java.net.InetAddress;
+
+import junit.framework.TestCase;
+
+public class NetworkUtilsTest extends TestCase {
+
+    private InetAddress Address(String addr) {
+        return InetAddress.parseNumericAddress(addr);
+    }
+
+    private Inet4Address IPv4Address(String addr) {
+        return (Inet4Address) Address(addr);
+    }
+
+    @SmallTest
+    public void testGetImplicitNetmask() {
+        assertEquals(8, NetworkUtils.getImplicitNetmask(IPv4Address("4.2.2.2")));
+        assertEquals(8, NetworkUtils.getImplicitNetmask(IPv4Address("10.5.6.7")));
+        assertEquals(16, NetworkUtils.getImplicitNetmask(IPv4Address("173.194.72.105")));
+        assertEquals(16, NetworkUtils.getImplicitNetmask(IPv4Address("172.23.68.145")));
+        assertEquals(24, NetworkUtils.getImplicitNetmask(IPv4Address("192.0.2.1")));
+        assertEquals(24, NetworkUtils.getImplicitNetmask(IPv4Address("192.168.5.1")));
+        assertEquals(32, NetworkUtils.getImplicitNetmask(IPv4Address("224.0.0.1")));
+        assertEquals(32, NetworkUtils.getImplicitNetmask(IPv4Address("255.6.7.8")));
+    }
+
+    private void assertInvalidNetworkMask(Inet4Address addr) {
+        try {
+            NetworkUtils.netmaskToPrefixLength(addr);
+            fail("Invalid netmask " + addr.getHostAddress() + " did not cause exception");
+        } catch (IllegalArgumentException expected) {
+        }
+    }
+
+    @SmallTest
+    public void testNetmaskToPrefixLength() {
+        assertEquals(0, NetworkUtils.netmaskToPrefixLength(IPv4Address("0.0.0.0")));
+        assertEquals(9, NetworkUtils.netmaskToPrefixLength(IPv4Address("255.128.0.0")));
+        assertEquals(17, NetworkUtils.netmaskToPrefixLength(IPv4Address("255.255.128.0")));
+        assertEquals(23, NetworkUtils.netmaskToPrefixLength(IPv4Address("255.255.254.0")));
+        assertEquals(31, NetworkUtils.netmaskToPrefixLength(IPv4Address("255.255.255.254")));
+        assertEquals(32, NetworkUtils.netmaskToPrefixLength(IPv4Address("255.255.255.255")));
+
+        assertInvalidNetworkMask(IPv4Address("0.0.0.1"));
+        assertInvalidNetworkMask(IPv4Address("255.255.255.253"));
+        assertInvalidNetworkMask(IPv4Address("255.255.0.255"));
+    }
+}