diff --git a/ryu/controller/network.py b/ryu/controller/network.py
index ac247ffd..9b9baa4e 100644
--- a/ryu/controller/network.py
+++ b/ryu/controller/network.py
@@ -18,12 +18,14 @@ import collections
 
 from ryu.base import app_manager
 import ryu.exception as ryu_exc
-from ryu.app.rest_nw_id import NW_ID_UNKNOWN
 from ryu.controller import event
 from ryu.exception import NetworkNotFound, NetworkAlreadyExist
 from ryu.exception import PortAlreadyExist, PortNotFound, PortUnknown
 
 
+NW_ID_UNKNOWN = '__NW_ID_UNKNOWN__'
+
+
 class MacAddressAlreadyExist(ryu_exc.RyuException):
     message = 'port (%(dpid)s, %(port)s) has already mac %(mac_address)s'