diff --git a/shadowsocks/common.py b/shadowsocks/common.py
index d022f3a..5c1bb76 100644
--- a/shadowsocks/common.py
+++ b/shadowsocks/common.py
@@ -265,7 +265,7 @@ class IPNetwork(object):
         list(map(self.add_network, addrs))
 
     def add_network(self, addr):
-        if addr is "":
+        if addr == "":
             return
         block = addr.split('/')
         addr_family = is_ip(block[0])
@@ -277,9 +277,9 @@ class IPNetwork(object):
             ip = (hi << 64) | lo
         else:
             raise Exception("Not a valid CIDR notation: %s" % addr)
-        if len(block) is 1:
+        if len(block) == 1:
             prefix_size = 0
-            while (ip & 1) == 0 and ip is not 0:
+            while (ip & 1) == 0 and ip != 0:
                 ip >>= 1
                 prefix_size += 1
             logging.warn("You did't specify CIDR routing prefix size for %s, "