NAT which is referred as Network Address Translation is used not only for solving the depletion of IPv4 address space but also to hide internal networks from outside world, hence further hardening the security when used with a firewall. Moreover its the NAT feature which allows us to use any address-space in our internal network complying with RFC 1918 of Private Address Space and Filtering.
Following Cisco PIX 500 series firewall, Cisco ASA 5500 series is not just a hardware firewall but provides firewall functionalities, antivirus, IDS/IPS engine and VPN/SSL capabilities. PIX stands for Private Internet eXchange while ASA stands for Adaptive Security Appliance.
A firewall is simply a physical box that inspects and filters traffic coming from one network to another network. Firewall isolates its interfaces from each other and control packets exchanging between those isolated interfaces. Firewall function is to protect the resources of internal network (home or office network) from unauthorized attacks from external network (Internet).
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