Covert Channels have existed for more than twenty years now. Although they did not receive a special attention in their early years, they are being more and more studied nowadays. This work focuses on network covert channels and it attempts to give an overview on their basics to later analyse several existing implementations which may compromise the security perimeter of a corporate network. The features under study are the bandwidth provided by the channel and the ease of detection. The studied tools have turned out to be in most cases unreliable and easy to detect with current detection techniques and the bandwidth provided is usually moderate but they might pose a threat if not taken into consideration.