In this paper, IDS (Intrusion Detection System) nodes are deployed in MANETs in order to mitigate black hole attacks. The IDS nodes must be set in sniff mode in order to perform the so-called ABM (Anti-Blackhole Mechanism) function, which is mainly used to estimate a suspicious value of a node according to the abnormal difference between the routing messages transmitted from the node. When a suspicious value exceeds a threshold, an IDS nearby will broadcast a block message, informing all nodes on the network, asking them to cooperatively isolate the malicious node.