The network is the entry point to your application. It provides the first gatekeepers that control access to the various servers in your environment. Servers are protected with their own operating system gatekeepers, but it is important not to allow them to be deluged with attacks from the network layer. It is equally important to ensure that network gatekeepers cannot be replaced or reconfigured by imposters. In a nutshell, network security involves protecting network devices and the data that they forward.

The Diagram below show the basic components of a network, which act as the front-line gatekeepers, are the router, the firewall, and the switch. 

The router is the very first line of defense. It provides packet routing, and it can also be configured to block or filter the forwarding of packet types that are known to be vulnerable or used maliciously, such as ICMP or Simple Network Management Protocol (SNMP).

If you don't have control of the router, there is little you can do to protect your network beyond asking your ISP what defense mechanisms they have in place on their routers.

The configuration categories for the router are:

• Patches and updates
• Protocols
• Administrative access
• Services
• Auditing and logging
• Intrusion detection

Patches and Updates

Subscribe to alert services provided by the manufacturer of your networking hardware so that you can stay current with both security issues and service patches. As vulnerabilities are found — and they inevitably will be found — good vendors make patches available quickly and announce these updates through e-mail or on their Web sites. Always test the updates before implementing them in a production environment.

Use Ingress and Egress Filtering

Spoofed packets are representative of probes, attacks, and a knowledgeable attacker. Incoming packets with an internal address can indicate an intrusion attempt or probe and should be denied entry to the perimeter network. Likewise, set up your router to route outgoing packets only if they have a valid internal IP address. Verifying outgoing packets does not protect you from a denial of service attack, but it does keep such attacks from originating from your network.