diff --git a/shadowsocks/common.py b/shadowsocks/common.py
index d582923..0f94e07 100644
--- a/shadowsocks/common.py
+++ b/shadowsocks/common.py
@@ -185,7 +185,7 @@ def parse_header(data):
 
 
 class IPNetwork(object):
-    ADDRLENGTH = {socket.AF_INET: 32, socket.AF_INET6: 128}
+    ADDRLENGTH = {socket.AF_INET: 32, socket.AF_INET6: 128, False: 0}
 
     def __init__(self, addrs):
         self._network_list_v4 = []
@@ -197,6 +197,7 @@ class IPNetwork(object):
     def add_network(self, addr):
         block = addr.split('/')
         addr_family = is_ip(block[0])
+        addr_len = IPNetwork.ADDRLENGTH[addr_family]
         if addr_family is socket.AF_INET:
             ip, = struct.unpack("!I", socket.inet_aton(block[0]))
         elif addr_family is socket.AF_INET6:
@@ -210,10 +211,9 @@ class IPNetwork(object):
                 ip >>= 1
                 prefix_size += 1
             logging.warn("You did't specify CIDR routing prefix size for %s, "
-                         "implicit treated as %s/%d" % (addr, addr,
-                         IPNetwork.ADDRLENGTH[addr_family] - prefix_size))
-        elif block[1].isdigit() and int(block[1]) <= IPNetwork.ADDRLENGTH[addr_family]:
-            prefix_size = IPNetwork.ADDRLENGTH[addr_family] - int(block[1])
+                         "implicit treated as %s/%d" % (addr, addr, addr_len))
+        elif block[1].isdigit() and int(block[1]) <= addr_len:
+            prefix_size = addr_len - int(block[1])
             ip >>= prefix_size
         else:
             raise SyntaxError("Not a valid CIDR notation: %s" % addr)
@@ -226,11 +226,13 @@ class IPNetwork(object):
         addr_family = is_ip(addr)
         if addr_family is socket.AF_INET:
             ip, = struct.unpack("!I", socket.inet_aton(addr))
-            return any(map(lambda (naddr, ps): naddr == ip >> ps, self._network_list_v4))
+            return any(map(lambda (n, ps): n == ip >> ps,
+                           self._network_list_v4))
         elif addr_family is socket.AF_INET6:
             hi, lo = struct.unpack("!QQ", inet_pton(addr_family, addr))
             ip = (hi << 64) | lo
-            return any(map(lambda (naddr, ps): naddr == ip >> ps, self._network_list_v6))
+            return any(map(lambda (n, ps): n == ip >> ps,
+                           self._network_list_v6))
         else:
             return False
 
@@ -262,7 +264,7 @@ def test_pack_header():
 
 
 def test_ip_network():
-    ip_network = IPNetwork('127.0.0.0/24,::ff:1/112,::1,192.168.1.1,192.168.2.0')
+    ip_network = IPNetwork('127.0.0.0/24,::ff:1/112,::1,192.168.1.1,192.0.2.0')
     assert '127.0.0.1' in ip_network
     assert '127.0.1.1' not in ip_network
     assert ':ff:ffff' in ip_network
@@ -271,8 +273,8 @@ def test_ip_network():
     assert '::2' not in ip_network
     assert '192.168.1.1' in ip_network
     assert '192.168.1.2' not in ip_network
-    assert '192.168.2.1' in ip_network
-    assert '192.168.3.1' in ip_network  # 192.168.2.0 is treated as 192.168.2.0/23
+    assert '192.0.2.1' in ip_network
+    assert '192.0.3.1' in ip_network  # 192.0.2.0 is treated as 192.0.2.0/23
     assert 'www.google.com' not in ip_network