The device suggested is a Network Intrusion Detection System (NIDS). A NIDS monitors network traffic for suspicious activity and alerts the system or network administrator. Unlike a Network Intrusion Prevention System (NIPS), which actively blocks traffic deemed malicious, a NIDS does not interfere with the flow of traffic, thus fulfilling the company's requirement for a device that only notifies rather than drops traffic.

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In the context of network sniffing, promiscuous mode is a specific setting for a network interface controller (NIC) that allows the NIC to pass all traffic it receives to the central processing unit, rather than just the frames that the NIC is specifically programmed to receive. This mode is essential for network sniffers, as it enables them to capture all network traffic, including packets not addressed to the NIC's MAC address. When a NIC is not in promiscuous mode, it will ignore packets not intended for its MAC address.
References: The explanation provided aligns with standard networking practices and the information available from various security resources, including the Information Security Stack Exchange1 and TechTarget2. These sources confirm the role of promiscuous mode in enabling a network device to intercept and read all network packets that arrive, which is necessary for the operation of network sniffers.

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