Corporations should perform Network Vulnerability assessments and internal PENTESTs on quarterly basis for security exposures.  Annually, a highly experienced security firm can perform more in-depth testing as needed.  Corporations must actively search for weaknesses in their security defenses, as the bad guys are actively engaged in the same process. It’s always better for security team to discover and mitigate these risks before any damages occur.

QUOTE: Proactive network security should be the norm rather than the exception, and to understand why, think about the risks: What would happen if your network or PCs went down for hours? Days? The answer could range from inaccessible files to a near-complete business standstill.  A network security audit follows nearly the same methodology as an attack. First, the attacker scans the network to determine IP addressing of networks and hosts. An attacker would start from the outside and work his way in by uncovering IP addresses from DNS queries. You’ve got a head start because you already know your IP addressing scheme; it’s just a matter of conducting a quick scan (also called a sweep) to determine which IP addresses are in use.

There are many ways to go through the audit. I like to use a combination of free and commercial tools. The best known free network scanning tools are Nmap and Nessus. Of those two, Nmap is easier to install and use, but Nessus has better reporting. Also check out McAfee’s SuperScan network scanning tool.Commercial tools I like include GFI LANguard and the eEye 1505 Security Management Appliance. If you’re willing to spend the money, in return you’ll get more information about each vulnerability and its remediation – not to mention more polished interfaces, more capabilities, and better reporting.