During the past five years, security of computer network has become mainstream in most of everyone's lives. Today, most discussions on computer security is centred on the tools or techniques used in protecting and defending networks. The aim of this paper is to examine the origins of detecting, analysing and reporting of malicious activity, where it is today and where it appears to be heading in the future. Some of the many techniques and tools presently used in Network defence will be explored as well. There are a variety of tools providing a certain level of comfort with acceptable risks used in the defence and surveillance of computer networks. Defence-in-Depth is a term encompassing comprehensive analyst training, hardware deployed in strategic positions and a strong security policy necessary for achieving this objective. Everyday, we have tools at our disposal to reach this goal. The aggregation of data comes from routers, the host itself, firewalls, virus scanners and a tool strictly designed to catch known attacks; an Intrusion Detection System (IDS).