Cisco Systems Configuring NAT (ASA 8.2 and Earlier)

CHAPTE R
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Cisco ASA Series Firewall ASDM Configuration Guide
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Configuring NAT (ASA 8.2 and Earlier)
This chapter describes Network Address Translation, and includes the following sections:
•NAT Overview, page6-1
•Configuring NAT Control, page6-16
•Using Dynamic NAT, page6-17
•Using Static NAT, page6-27
•Using NAT Exemption, page6-33
NAT Overview
This section describes how NAT works on the ASA, and includes the following topics:
•Introduction to NAT, page6-1
•NAT in Routed Mode, page6-2
•NAT in Transparent Mode, page6-3
•NAT Control, page6-4
•NAT Types, page6-6
•Policy NAT, page6-11
•NAT and Same Security Level Interfaces, page6-13
•Order of NAT Rules Used to Match Real Addresses, page6-14
•Mapped Address Guidelines, page6-14
•DNS and NAT, page6-14
Introduction to NAT
Address translation substitutes the real address in a packet with a mapped address that is routable on the
destination network. NAT is composed of two steps: the process by which a real address is translated
into a mapped address, and the process to undo translation for returning traffic.
The ASA translates an address when a NAT rule matches the traffic. If no NAT rule matches, processing
for the packet continues. The exception is when you enable NAT control. NATcontrol requires that
packets traversing from a higher security interface (inside) to a lower security interface (outside) match
a NAT rule, or processing for the packet stops. See the “Security Levels” section on page 13-1 in the