If you compile RIOT for the native cpu and include the native_net
module, you need to specify a network interface like this:
./bin/default-native.elf tap0


SETTING UP A TAP NETWORK
========================

There is a shellscript in RIOT/cpu/native called tapsetup.sh which you
can use to create a network of tap interfaces.

Usage:
To create a bridge and two (or count at your option) tap interfaces:
./tapsetup.sh create [count]
To delete the bridge and all tap interfaces:
./tapsetup.sh delete


OSX
===

For tun/tap networking in OSX you will need:
http://tuntaposx.sourceforge.net/

For OSX there is a seperate script called tapsetup-osx.sh.
Run it, (it instructs you to start the RIOT instances).
In contrast to linux you will need to run 'tapsetup-osx.sh delete'
after killing your instances and rerun 'tapsetup-osx.sh create' before
restarting.
Packet delivery under OSX only works with user assistance at the
moment. run 'kill -SIGIO <RIOT process ID>' to deliver a packet to a
specific RIOT instance.