Cisco Systems NAT Rule Order

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Cisco ASA Series Firewall ASDM Configuration Guide
Chapter3 Information About NAT (ASA 8.3 and Later)
NAT Rule Order
NAT Rule Order
Network object NAT rules and twice NAT rules are stored in a single table that is divided into three
sections. Section 1 rules are applied first, then section 2, and finally section 3, until a match is found.
For example, if a match is found in section 1, sections 2 and 3 are not evaluated. Table 3- 1 shows the
order of rules within each section.
Table3-1 NAT Rule Table
Table Section Rule Type Order of Rules within the Section
Section 1 Twice NAT Applied on a first match basis, in the order they appear in the
configuration. Because the first match is applied, you must
ensure that specific rules come before more general rules, or
the specific rules might not be applied as desired. By default,
twice NAT rules are added to section1.
Note If you configure EasyVPN remote, the ASA
dynamically adds invisible NAT rules to the end of this
section. Be sure that you do not configure a twice NAT
rule in this section that might match your VPN traffic,
instead of matching the invisible rule. If VPN does not
work due to NAT failure, consider adding twice NAT
rules to section 3 instead.
Section 2 Network object NAT If a match in section 1 is not found, section 2 rules are applied
in the following order, as automatically determined by the
ASA:
1. Static rules.
2. Dynamic rules.
Within each rule type, the following ordering guidelines are
used:
a. Quantity of real IP addresses—From smallest to
largest. For example, an object with one address will
be assessed before an object with 10 addresses.
b. For quantities that are the same, then the IP address
number is used, from lowest to highest. For example,
10.1.1.0 is assessed before 11.1.1.0.
c. If the same IP address is used, then the name of the
network object is used, in alphabetical order. For
example, abracadabra is assessed before catwoman.
Section 3 Twice NAT If a match is still not found, section 3 rules are applied on a first
match basis, in the order they appear in the configuration. This
section should contain your most general rules. You must also
ensure that any specific rules in this section come before
general rules that would otherwise apply. You can specify
whether to add a twice NAT rule to section 3 when you add the
rule.