CHAPTER
26-1
Cisco ASA 5500 Series Configuration Guide using ASDM
OL-20339-01
26
Information About NAT
This chapter provides an overview of how Network Address Translation (NAT) works on the adaptive
security appliance. This chapter includes the following sections:
• Why Use NAT?, page 26-1
• NAT Terminology, page 26-2
• NAT Types, page 26-2
• NAT in Routed and Transparent Mode, page 26-12
• How NAT is Implemented, page 26-15
• NAT Rule Order, page 26-19
• Mapped Address Guidelines, page 26-20
• DNS and NAT, page 26-21
• Where to Go Next, page 26-23
Note To start configuring NAT, see Chapter 27, “Configuring Network Object NAT,” or Chapter 28,
“Configuring Twice NAT.”
Why Use NAT?
Each computer and device within an IP network is assigned a unique IP address that identifies the host.
Because of a shortage of public IPv4 addresses, most of these IP addresses are private, not routable
anywhere outside of the private company network. RFC 1918 defines the private IP addresses you can
use internally (Table 26-1):
Table 26-1 Private IP Addresses
Network Class Address Block Starting Address Ending Address Approximate Hosts
Class A addresses 10.0.0.0/8 10.0.0.0 10.255.255.255 16,000,000
Class B addresses 172.16.0.0/12 172.16.0.0 172.31.255.255 1,000,000
Class C addresses 192.168.0.0/16 192.168.0.0 192.168.255.255 65,000
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