Layered Security: Intrusion Detection System
Installing an intrusion detection system (IDS), an increasingly common component of layered security, will "raise the bar" even higher on the level of determination needed by an attacker to effectively penetrate your network. An IDS will:

• Augment current access control systems, such as firewalls, by warning system administrators of real and attempted intrusions.
  
• Recognize attacks against the network that firewalls often can't see--those that are coming from the inside of the organization.
  
• Provide information to assist in damage control or mitigation after an intrusion attempt.

Other Elements of Layered Security
Even if the network IDS fails, firewalls, vulnerability assessment, and antivirus software are still effective. A properly configured firewall can block most unsophisticated attacks before a network IDS sees it. Vulnerability assessment can identify weaknesses and help to eliminate them. If a system has no vulnerabilities, then real attacks fail even if they are not detected. Even if the IDS does not detect the delivery of known viruses, the firewall does not block their delivery, and the vulnerability assessment does not eliminate the means of infection, antivirus software can still detect known viruses.