Cisco IOS® Network Address Translation (NAT) is designed for IP address
simplification and conservation. It enables private IP internetworks that use
nonregistered IP addresses to connect to the Internet. NAT operates on a Cisco
router that connects two networks together, and translates the private (inside
local) addresses in the internal network to public addresses (outside local)
before packets are forwarded to another network. As a part of this
functionality, you can configure NAT to advertise only one address for the
entire network to the outside world. This effectively hides the internal
network from the world. Therefore, it provides additional security.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Technical Tips Conventions.
One of the main features of NAT is static Port Address Translation
(PAT), which is also referred to as "overload" in a Cisco IOS configuration.
Static PAT is designed to allow one-to-one mapping between local and global
addresses. A common use for static PAT is to allow Internet users from the
public network to access a Web server located in the private network.
In order to get more information about NAT, refer to the
Technical Support pages.
This table shows the three blocks of IP address space available for
private networks. Consult
more details about these special networks.
IP Address Space
10.0.0.0 - 10.255.255.255 (10/8 prefix
172.16.0.0 - 172.31.255.255 (172.16/12 prefix
192.168.0.0 - 192.168.255.255 (192.168/16
Note: The first block is nothing but a single class A network number, while
the second block is a set of 16 contiguous class B network numbers, and third
block is a set of 256 contiguous class C network numbers.
In this example, the Internet Service Provider (ISP) assigns the DSL
subscriber only a single IP address, 18.104.22.168/24. The assigned IP address is
a registered unique IP address and is called an inside global address. This
registered IP address is used by the entire private network to browse the
Internet and also by Internet users that come from the public network to reach
the Web server in the private network.
The private LAN, 192.168.0.0/24, is connected to the Ethernet interface
of the NAT router. This private LAN contains several PCs and a Web server. The
NAT router is configured to translate the unregistered IP addresses (inside
local addresses) that come from these PCs to a single public IP address (inside
global - 22.214.171.124) to browse the Internet.
IP address 192.168.0.5 (Web server) is an address in the private
address space that cannot be routed to the Internet. The only visible IP
address for public Internet users to reach the Web server is 126.96.36.199.
Therefore, the NAT router is configured to perform a one-to-one mapping between
IP address 188.8.131.52 port 80 (port 80 is used to browse the Internet) and
192.168.0.5 port 80. This mapping allows Internet users on the public side to
have access to the internal Web server.
This network topology and sample configuration can be used for the
Cisco 827, 1417, SOHO77, and 1700/2600/3600 ADSL WIC. As an example, the Cisco
827 is used in this document.
In this section, you are presented with the information you can use to
configure the features described in this document.
Note: In order to find additional information on the commands used in this
document, refer to the IOS
Command Lookup tool
(registered customers only)
This document uses this network setup.
service timestamps debug uptime
service timestamps log uptime
no ip domain-lookup
ip address 192.168.0.254 255.255.255.0
ip nat inside
!--- This is the inside local IP address and it is a private IP address.
no ip address
no atm ilmi-keepalive
dsl operating-mode auto
ip address 184.108.40.206 255.255.255.240
ip nat outside
!--- This is the inside global IP address.
!--- This is your public IP address and it is provided to you by your ISP.
ip nat inside source list 1 interface BVI1 overload
!--- This statement makes the router perform PAT for all the
!--- End Stations behind the Ethernet interface that uses
!--- private IP addresses defined in access list #1.
ip nat inside source static tcp 192.168.0.5 80 220.127.116.11 80 extendable
!--- This statement performs the static address translation for the Web server.
!--- With this statement, users that try to reach 18.104.22.168 port 80 (www) are
!--- automatically redirected to 192.168.0.5 port 80 (www). In this case
!--- it is the Web server.
ip route 0.0.0.0 0.0.0.0 22.214.171.124
!--- IP address 126.96.36.199 is the next hop IP address, also
!--- called the default gateway.
!--- Your ISP can tell you what IP address to configure as the next hop address.
access-list 1 permit 192.168.0.0 0.0.0.255
!--- This access list defines the private network
!--- that is network address translated.
bridge 1 protocol ieee
bridge 1 route ip
From the show ip nat translation command
output, the Inside local is the configured IP address assigned to the Web
server on the inside network. Notice that 192.168.0.5 is an address in the
private address space that cannot be routed to the Internet. The Inside global
is the IP address of the inside host, which is the Web server, as it appears to
the outside network. This address is the one known to people who try to access
the Web server from the Internet.
The Outside local is the IP address of
the outside host as it appears to the inside network. It is not necessarily a
legitimate address. But, it is allocated from an address space that can be
routed on the inside.
The Outside global address is the IP
address assigned to a host on the outside network by the owner of the host. The
address is allocated from an address or network space that can be globally
Notice that the address 188.8.131.52 with port number 80 (HTTP)
translates to 192.168.0.5 port 80, and vice versa. Therefore, Internet users
can browse the Web server even though the Web server is on a private network
with a private IP address.
In order to get more information about how to troubleshoot NAT, refer
NAT Operation and Basic NAT Troubleshooting.
827#show ip nat translation
Pro Inside global Inside local Outside local Outside global
tcp 184.108.40.206:80 192.168.0.5:80 --- ---
tcp 220.127.116.11:80 192.168.0.5:80 18.104.22.168:11000 22.214.171.124:11000
In order to troubleshoot address translation, you can issue
the term mon and debug ip nat detailed
commands on the router to see if the address translates
correctly. The visible IP address for outside users to reach the Web server is
126.96.36.199. For example, users from the public side of the Internet who try to
reach 188.8.131.52 port 80 (www) are automatically redirected to 192.168.0.5 port
80 (www), which in this case is the Web server.
827#debug ip nat detailed
IP NAT detailed debugging is on
03:29:49: NAT: creating portlist proto 6 globaladdr 184.108.40.206
03:29:49: NAT: Allocated Port for 192.168.0.5 -> 220.127.116.11: wanted 80 got 80
03:29:49: NAT: o: tcp (18.104.22.168, 11000) -> (22.214.171.124, 80) 
<... snipped ...>