Depends on the network. Cisco routers support Access Control Lists (ACLs) for perimeter/border security; Cisco PIX supports Security Policies for firewall (or 2nd level of defense) security ? both support Secure Shell (SSH) encryption and anti-spoofing and anti-Denial of Service (DoS) attacks, to name a few options. Servers support TCP/IP filtering and auditing. Application layer firewalls further compliment network firewalls and Intrusion Detection Systems (IDS). Desktop firewall also compliment network firewalls. Read through my series for security tips on improving network security.
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