A NIDS needs to have access to all packets traveling around a network. In general, every packet passing over a network visits every device on it – it is just that the network adapter on each device ignores packets that aren’t addressed to it. So, the NIDS can be hosted on any device on the network just as long as that device’s network adapter has been placed in promiscuous mode to pay attention to all packets, not just those addressed to that device. If the network has several separate segments, such as with a demilitarized zone configuration, you can either host the NIDS on a connecting device, such as the router, or place a collector on the other segment that will communicate with the processing module resident on a host connected to the main network. A NIDS can also examine packets on a remote network if it has an agent installed there.