diff --git a/app/src/main/java/eu/faircode/netguard/IPUtil.java b/app/src/main/java/eu/faircode/netguard/IPUtil.java
new file mode 100644
index 00000000..29bff112
--- /dev/null
+++ b/app/src/main/java/eu/faircode/netguard/IPUtil.java
@@ -0,0 +1,101 @@
+package eu.faircode.netguard;
+
+/*
+    This file is part of NetGuard.
+
+    NetGuard is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    NetGuard is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with NetGuard.  If not, see <http://www.gnu.org/licenses/>.
+
+    Copyright 2015-2016 by Marcel Bokhorst (M66B)
+*/
+
+import android.util.Log;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+import java.util.ArrayList;
+import java.util.List;
+
+public class IPUtil {
+    private static final String TAG = "NetGuard.IPUtil";
+
+    public static List<CIDR> toCIDR(String start, String end) throws UnknownHostException {
+        return toCIDR(InetAddress.getByName(start), InetAddress.getByName(end));
+    }
+
+    public static List<CIDR> toCIDR(InetAddress start, InetAddress end) throws UnknownHostException {
+        List<CIDR> listResult = new ArrayList<>();
+
+        Log.i(TAG, "toCIDR(" + start.getHostAddress() + "," + end.getHostAddress() + ")");
+
+        long from = inet2long(start);
+        long to = inet2long(end);
+        while (to >= from) {
+            byte prefix = 32;
+            while (prefix > 0) {
+                long mask = (0xFFFFFFFF00000000L >> (prefix - 1)) % 0x0100000000L;
+                if ((from & mask) != from)
+                    break;
+                prefix--;
+            }
+
+            byte max = (byte) (32 - Math.floor(Math.log(to - from + 1) / Math.log(2)));
+            if (prefix < max)
+                prefix = max;
+
+            listResult.add(new CIDR(long2inet(from), prefix));
+
+            from += Math.pow(2, (32 - prefix));
+        }
+
+        for (CIDR cidr : listResult)
+            Log.i(TAG, cidr.toString());
+
+        return listResult;
+    }
+
+    private static long inet2long(InetAddress addr) {
+        long result = 0;
+        for (byte b : addr.getAddress())
+            result = result << 8 | (b & 0xFF);
+        return result;
+    }
+
+    private static InetAddress long2inet(long addr) throws UnknownHostException {
+        byte[] b = new byte[4];
+        for (int i = b.length - 1; i >= 0; i--) {
+            b[i] = (byte) (addr & 0xFF);
+            addr = addr >> 8;
+        }
+        return InetAddress.getByAddress(b);
+    }
+
+    public static String minus1(String addr) throws UnknownHostException {
+        return long2inet(inet2long(InetAddress.getByName(addr)) - 1).getHostAddress();
+    }
+
+    public static class CIDR {
+        public InetAddress address;
+        public int prefix;
+
+        public CIDR(InetAddress address, int prefix) {
+            this.address = address;
+            this.prefix = prefix;
+        }
+
+        @Override
+        public String toString() {
+            return address.getHostAddress() + "/" + prefix;
+        }
+    }
+}