The external interface makes it possible to connect the simulation with real 
networks. 
In this example the network consists of eight clients, four switches,
three routers, and an extRouter, that connects the simulation 
to a real host. 
This network can be used to apply traceroute, a tool to trace the 
path through the network to a specified host. 

To run this examples, some parameters have to be adjusted in the omnetpp.ini.

- Set **.ext[0].device to the device name of the interface of the computer 
  running the simulation.
- On your server, add a route to the nets 10.2. and 10.3., and set 
  the gateway to the IP address of the computer, on which the simulation is 
  running.

Start the simulation with root priviledges. To get realistic round trip times 
you should choose the express mode. 
Type traceroute 10.2.1.1 at the command prompt of the external machine.






