diff --git a/model/fib/ndn-fib-impl.h b/model/fib/ndn-fib-impl.h
index 4c78997..1efead9 100644
--- a/model/fib/ndn-fib-impl.h
+++ b/model/fib/ndn-fib-impl.h
@@ -110,13 +110,22 @@
   GetSize () const;
 
   virtual Ptr<const Entry>
+  Begin () const;
+
+  virtual Ptr<Entry>
   Begin ();
 
   virtual Ptr<const Entry>
+  End () const;
+
+  virtual Ptr<Entry>
   End ();
 
   virtual Ptr<const Entry>
-  Next (Ptr<const Entry> item);
+  Next (Ptr<const Entry> item) const;
+  
+  virtual Ptr<Entry>
+  Next (Ptr<Entry> item);
   
 protected:
   // inherited from Object class