This paper presents the methodologies of IDS technologies. Essentially, any system requiring security must be protected from attacks. Intrusiondetection systems are used to detect unusual activity in a network of computer systems to identify if activity is unfriendly or unauthorized in order to enable a response to that violation. To achieve this, there are two main types of IDS: network-based and host-based. This paper outlines these two types of IDS respectively and highlights the advantages of each kind.