Static Nat - Cisco FirePOWER ASA 5500 series Configuration Manual

Chapter 17 Applying NAT
Dynamic NAT has these disadvantages:
•
•
The advantage of dynamic NAT is that some protocols cannot use PAT. For example, PAT does not work
with IP protocols that do not have a port to overload, such as GRE version 0. PAT also does not work
with some applications that have a data stream on one port and the control path on another and are not
open standard, such as some multimedia applications. See the
Inspection" section on page 25-2
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 security appliance does not create a translation at all unless the
translated host is the initiator. See the following
to hosts.
PAT lets you use a single mapped address, thus conserving routable addresses. You can even use the
security appliance 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
Application Protocol Inspection" section on page 25-2
support.
For the duration of the translation, a remote host can initiate a connection to the translated host if an
Note
access list allows it. Because the port address (both real and mapped) is unpredictable, a connection to
the host is unlikely. However in this case, you can rely on the security of the access list.

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 there
is an access list that allows it).
The main difference between dynamic NAT and a range of addresses for static NAT is that static NAT
allows a remote host to initiate a connection to a translated host (if there is an access list that allows it),
while dynamic NAT does not. You also need an equal number of mapped addresses as real addresses with
static NAT.
OL-10088-01
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.
for more information about NAT and PAT support.
"Static NAT"
for more information about NAT and PAT
Cisco Security Appliance Command Line Configuration Guide
"When to Use Application Protocol
or "Static PAT" sections for reliable access
"When to Use
NAT Overview
17-7