When you add functionality, you add vulnerability. This is true whether your network powers a small business, or a Fortune 500 company. This is true whether you are looking at a single Lan, or a world-wide Wan. Every piece you add brings with it risk.
We use controls to mitigate risk. Mainly these are directive, preventative, and detective controls. Directive or policy controls define appropriate use of the system. Sometimes people will follow these policies, sometimes they will not. Of course, technology allows us to set permissions and therefore implement preventative controls to keep people from doing certain things. For the times when people do not follow policy and the system does not (or cannot) stop them, detective controls are there to leave an audit trail.
When we add functionality, we need to consider these controls. Now let us work our way from cables to computers.
When deploying a network switch and cabling, ask, how could this break? What can we do to protect it? Consider fault-tolerant designs such as full-mesh for connecting multiple switches and cabling with Fibre instead of copper. Consider directive controls to specify who can access the switch and how. Consider preventative controls such as locked rooms and climate controls. Consider detective controls such as check-in forms and electronic monitoring.
What are Ethernet’s risks and what controls are available? Directive controls could include specifying how a computer gets connected to the Ethernet Lan. It could also specify who is allowed to run sniffers. The preventative controls would be locking down the switch ports to specific Mac addresses, thereby making it difficult to connect a computer without first following procedure. Consider enabling the switch manufacturer’s protection against ARP cache poisoning, such as Cisco’s Dynamic ARP Inspection. Detective controls enabling the switch’s audit logs, and regularly scanning for network cards in promiscuous mode with tools like Sentinel.
What about IP’s risks and controls? Design IP networks to address concerns about addressing and fragmentation. Use directive controls to specify who can access what IP and Internet resources, and who is responsible for addressing and naming. Preventative controls include securing Dhcp address servers and Wins/Dns name servers. Detective controls include auditing and logging. Monitor for IP address and name conflicts, and excessive traffic to Wins/Dns.
Good design along with proper controls will protect integrity and availability. All along, we have been assuming that there is no confidentiality. We have suggested that the place for this is in applications (application, presentation, session, transport layers). Ask your vendors for recommendations on how to securely run their applications. First build a solid network infrastructure, and then turn your attention towards securely delivering applications over it.