--- a/src/internet-stack/ipv4-l3-protocol.h	Sun Oct 12 18:04:10 2008 +0100
+++ b/src/internet-stack/ipv4-l3-protocol.h	Wed Oct 15 09:49:39 2008 +0200
@@ -80,7 +80,7 @@
    * to forward packets up the stack to the right protocol.
    * It is also called from NodeImpl::GetUdp for example.
    */
-  Ptr<Ipv4L4Protocol> GetProtocol(int protocolNumber);
+  Ptr<Ipv4L4Protocol> GetProtocol(int protocolNumber) const;
   /**
    * \param protocol protocol to remove from this demux.
    *