diff --git a/src/network/bmproto.py b/src/network/bmproto.py
index bf0b5742..11e96fd6 100644
--- a/src/network/bmproto.py
+++ b/src/network/bmproto.py
@@ -71,6 +71,8 @@ class BMProto(AdvancedDispatcher, ObjectTracker):
         # packet/connection from a local IP
         self.local = False
         self.pendingUpload = RandomTrackingDict()
+        # canonical identifier of network group
+        self.network_group = None
 
     def bm_proto_reset(self):
         """Reset the bitmessage object parser"""
diff --git a/src/network/connectionpool.py b/src/network/connectionpool.py
index 654b74a1..6264191d 100644
--- a/src/network/connectionpool.py
+++ b/src/network/connectionpool.py
@@ -229,6 +229,7 @@ class BMConnectionPool(object):
 
     def loop(self):  # pylint: disable=too-many-branches,too-many-statements
         """Main Connectionpool's loop"""
+        # pylint: disable=too-many-locals
         # defaults to empty loop if outbound connections are maxed
         spawnConnections = False
         acceptConnections = True
@@ -297,6 +298,19 @@ class BMConnectionPool(object):
                     # don't connect to self
                     if chosen in state.ownAddresses:
                         continue
+                    # don't connect to the hosts from the same
+                    # network group, defense against sibyl attacks
+                    host_network_group = protocol.network_group(
+                        chosen.host)
+                    same_group = False
+                    for j in self.outboundConnections.values():
+                        if host_network_group == j.network_group:
+                            same_group = True
+                            if chosen.host == j.destination.host:
+                                knownnodes.decreaseRating(chosen)
+                            break
+                    if same_group:
+                        continue
 
                     try:
                         if chosen.host.endswith(".onion") and Proxy.onion_proxy:
diff --git a/src/network/tcp.py b/src/network/tcp.py
index 97b00784..31d20dea 100644
--- a/src/network/tcp.py
+++ b/src/network/tcp.py
@@ -84,6 +84,7 @@ class TCPConnection(BMProto, TLSDispatcher):
             )
         except socket.error:
             pass  # it's probably a hostname
+        self.network_group = protocol.network_group(self.destination.host)
         ObjectTracker.__init__(self)  # pylint: disable=non-parent-init-called
         self.bm_proto_reset()
         self.set_state("bm_header", expectBytes=protocol.Header.size)
diff --git a/src/protocol.py b/src/protocol.py
index ef101a72..ec8fc9dd 100644
--- a/src/protocol.py
+++ b/src/protocol.py
@@ -105,6 +105,34 @@ def networkType(host):
     return 'IPv6'
 
 
+def network_group(host):
+    """Canonical identifier of network group
+       simplified, borrowed from
+       GetGroup() in src/netaddresses.cpp in bitcoin core"""
+    if not isinstance(host, str):
+        return None
+    network_type = networkType(host)
+    try:
+        raw_host = encodeHost(host)
+    except socket.error:
+        return host
+    if network_type == 'IPv4':
+        decoded_host = checkIPv4Address(raw_host[12:], True)
+        if decoded_host:
+            # /16 subnet
+            return raw_host[12:14]
+    elif network_type == 'IPv6':
+        decoded_host = checkIPv6Address(raw_host, True)
+        if decoded_host:
+            # /32 subnet
+            return raw_host[0:12]
+    else:
+        # just host, e.g. for tor
+        return host
+    # global network type group for local, private, unroutable
+    return network_type
+
+
 def checkIPAddress(host, private=False):
     """Returns hostStandardFormat if it is a valid IP address, otherwise returns False"""
     if host[0:12] == '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xFF\xFF':
diff --git a/src/tests/test_networkgroup.py b/src/tests/test_networkgroup.py
new file mode 100644
index 00000000..76cfb033
--- /dev/null
+++ b/src/tests/test_networkgroup.py
@@ -0,0 +1,39 @@
+"""
+Test for network group
+"""
+import unittest
+
+
+class TestNetworkGroup(unittest.TestCase):
+    """
+    Test case for network group
+    """
+    def test_network_group(self):
+        """Test various types of network groups"""
+        from pybitmessage.protocol import network_group
+
+        test_ip = '1.2.3.4'
+        self.assertEqual('\x01\x02', network_group(test_ip))
+
+        test_ip = '127.0.0.1'
+        self.assertEqual('IPv4', network_group(test_ip))
+
+        test_ip = '0102:0304:0506:0708:090A:0B0C:0D0E:0F10'
+        self.assertEqual(
+            '\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0A\x0B\x0C',
+            network_group(test_ip))
+
+        test_ip = 'bootstrap8444.bitmessage.org'
+        self.assertEqual(
+            'bootstrap8444.bitmessage.org',
+            network_group(test_ip))
+
+        test_ip = 'quzwelsuziwqgpt2.onion'
+        self.assertEqual(
+            test_ip,
+            network_group(test_ip))
+
+        test_ip = None
+        self.assertEqual(
+            None,
+            network_group(test_ip))