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Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter 35 Configuring NAT (ASA 8.2 and Earlier)
NAT Overview
Dynamic NAT has these disadvantages:
• If the mapped pool has fewer addresses than the real group, you could run out of addresses if the
amount of traffic is more than expected.
Use PAT if this event occurs often, because PAT provides over 64,000 translations using ports of a
single address.
• You have to use a large number of routable addresses in the mapped pool; if the destination network
requires registered addresses, such as the Internet, you might encounter a shortage of usable
addresses.
The advantage of dynamic NAT is that some protocols cannot use PAT. PAT does not work with the
following:
• IP protocols that do not have a port to overload, such as GRE version 0.
• Some multimedia applications that have a data stream on one port, the control path on another port,
and are not open standard.
See the “When to Use Application Protocol Inspection” section on page 46-2 for more information about
NAT and PAT support.
PAT
PAT translates multiple real addresses to a single mapped IP address. Specifically, the security appliance
translates the real address and source port (real socket) to the mapped address and a unique port above
1024 (mapped socket). Each connection requires a separate translation, because the source port differs
for each connection. For example, 10.1.1.1:1025 requires a separate translation from 10.1.1.1:1026.
After the connection expires, the port translation also expires after 30 seconds of inactivity. The timeout
is not configurable. Users on the destination network cannot reliably initiate a connection to a host that
uses PAT (even if the connection is allowed by an access list). Not only can you not predict the real or
mapped port number of the host, but the ASA does not create a translation at all unless the translated
host is the initiator. See the following “Static NAT” or “Static PAT” sections for reliable access to hosts.
PAT lets you use a single mapped address, thus conserving routable addresses. You can even use the ASA
interface IP address as the PAT address. PAT does not work with some multimedia applications that have
a data stream that is different from the control path. See the “When to Use Application Protocol
Inspection” section on page 46-2 for more information about NAT and PAT support.
Note For the duration of the translation, a remote host can initiate a connection to the translated host if an
access list allows it. Because the port address (both real and mapped) is unpredictable, a connection to
the host is unlikely. Nevertheless, in this case, you can rely on the security of the access list. However,
policy PAT does not support time-based ACLs.
Static NAT
Static NAT creates a fixed translation of real address(es) to mapped address(es).With dynamic NAT and
PAT, each host uses a different address or port for each subsequent translation. Because the mapped
address is the same for each consecutive connection with static NAT, and a persistent translation rule
exists, static NAT allows hosts on the destination network to initiate traffic to a translated host (if an
access list exists that allows it).