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Contiki network I/O on Microsoft Windows (including the Cygwin environment) is implemented based on the quite popular WinPcap library that is available at http://winpcap.org.
Developing Contiki network applications most likely involves working with a network protocol analyzer. Wireshark (formerly known as Ethereal) is a very popular one that on Windows uses - and actually comes with - the WinPcap libary. Wireshark is available at http://wireshark.org.
So with Wireshark installed Contiki network I/O doesn't need any additional components.
On Windows every Contiki application has one obligatory comand line argument that identifies the Windows network interface to be used by Contiki. While on Unix those network interfaces are called i.e. '/dev/tap0' they have on Windows names like
\Device\NPF_{F76B480A-1D31-4B3D-8002-C0EF49185737}
In order to avoid the necessity to enter such names on the command line instead the IPv4 address used by Windows is entered to identify the network interface to be used by Contiki. Please note that this IPv4 address is NOT the IPv4 address to be used by Contiki !
Contiki network I/O on Windows uses the same MAC address used by Windows. This approach often described as IP-Aliasing was primarily chosen because it avoids putting the network interface into promiscuous mode. The major benefit of this is the compatibility with WLAN interfaces - which mostly come with Windows device drivers incapable of promiscuous mode.
The WinPcap library works fine with the 'Microsoft Loopback Adapter' so it's easy to have a Contiki network application running on Windows communicate with the local Windows instance for testing purposes - and monitor the communication with Wireshark.
Windows Vista however tries to identify networks by the MAC address of the default router. If that fails the network is defined as an 'Unidentified Network' and thus classified as 'Public Network' resulting in very strict firewall settings. As there's no default router for a loopback interface the interface is always considered as a public network - which is kind of the opposite of the actual situation ;-)
Instead of fiddling with the firewall settings for 'Public Networks' (or even turning the firewall completely off) there's a clean solution which defines the loopback interface as not a true network interface that connects to a network. This results in generally deactivating both the network identification process and the firewall for the loopback interface. The details are available in the Microsoft TechNet Forums thread 'Vista Network Identification for Loopback Adpater' that is currently available at link