I' forgot to give some details :
I don't want to put my m0n0wall device between my DSL box and my switch because I don't want to have all my Internet traffic cross this device.
So, I just want to redirect the ports used for pptp to my m0n0wall device wich will be on my switch like all my other devices. So what I don't know is the way to plug it.
Hope my request is understandable.
Thanks in advance for your help.