Network Address
Translation (NAT)
At the end of Chapter 2, “Internet Protocols” of the Sybex CCNA
Study Guide Standard and CCNA Study Guide Deluxe Editions,
I provided a section called “Introduction to Network Address
Translation.” In it, I explained some basic terms, but I’ve been hearing from readers that it
doesn’t have enough information. I really have to expand on Network Address Translation
(NAT) in order to equip you with a thorough understanding of this very important topic.
So what’s new here? Well, in this update, I’m going to give you the skinny on NAT, Dynamic
NAT, and Port Address Translation (PAT)—also known as NAT Overload—in a lot more
detail. And I’m going to finish this update with an important hands-on lab so you can test your
understanding of these topics.
I’m giving you this update with the assumption that you’ve read at least
through Chapter 6, “Enhanced IGRP (EIGRP) and Open Shortest Path First
(OSPF),” of the CCNA Study Guide. Of course, it will be even better if you’ve
read the entire book!
So… When Do We Use NAT?
NAT, at times, decreases the overwhelming amount of Public IP addresses required in your net-
working environment. And NAT comes in really handy when two companies that have dupli-
cate internal addressing schemes merge. NAT is also great to have around when an organization
changes its Internet Service Provider (ISP) and the networking manager doesn’t want to hassle
with changing the internal address scheme.
Here’s a list of situations when it’s best to have NAT on your side:
You need to connect to the Internet and your hosts don’t have globally unique IP addresses.
You change to a new ISP that requires you to renumber your network.
You require two intranets with duplicate addresses to merge.
You typically use NAT on a border router. For an illustration of this, check out Figure 1.1.
Okay—so yeah. NAT’s totally cool. It’s the grooviest greatest network gadget, and you just
gotta have it—right? Well, hang on a minute. There are truly some serious snags related to NAT
use. Oh—don’t get me wrong. It really can save you sometimes, but there’s a dark side you need
to know about too! To give you a visual of the pros and cons linked to using NAT, check out
Table 1.1.
Types of Network Address Translation 3
FIGURE 1.1 Where to configure NAT
Corporate Engineering Sales
Internet
TABLE 1.1 Advantages and Disadvantages of Implementing NAT
Advantages Disadvantages
Conserves legally registered addresses Translation introduces switching path delays
Reduces address overlap occurrence Loss of end-to-end IP traceability
Increases flexibility when connecting to Certain applications will not function with
Internet NAT enabled.
Eliminates address renumbering as
network changes
The most obvious advantage associated with NAT is that it allows you to con-
serve your legally registered address scheme. And by the way, this is the rea-
son we haven’t run out of IPv4 addresses—think about it.
Types of Network Address Translation
Next, I’m going to go over the three types of NAT with you:
Static NAT This type of NAT is designed to allow one-to-one mapping between local and
global addresses. Keep in mind that the static flavor requires that you have one real Internet
IP address for every host on your network.
4 Network Address Translation (NAT)
Dynamic NAT This version gives you the ability to map an unregistered IP address to a reg-
istered IP address from out of a pool of registered IP addresses. You don’t have to statically con-
figure your router to map an inside to an outside address like you would using static NAT, but
you do have to have enough real, bona-fide IP addresses for everyone who’s going to be sending
packets to and from the Internet.
Overloading Okay—here it is people—the most popular type of NAT configuration! Under-
stand that overloading really is a form of dynamic NAT that maps multiple unregistered IP
addresses to a single registered IP address—many-to-one—by using different ports. Now, why
is this so special? Well, because it’s also known as Port Address Translation, or, PAT. And by
using PAT, (NAT Overload), you get to have thousands of users connect to the Internet using
only one real global IP address—pretty slick, yeah? Seriously, NAT Overload is truly the reason
we haven’t run out of valid IP address on the Internet! Really—I’m not joking.
No worries… I’ll show you how to configure all three types of NAT in the hands-on
lab at the end of this update!
NAT Names
The names we use to describe the addresses used with NAT are pretty simple. Addresses used
after NAT translations are called global addresses. These are usually the public addresses used
on the Internet, but remember, you don’t need public addresses if you aren’t going on the Inter-
net—right?
Local addresses are the ones we use before NAT translation. So, the inside local address is
actually the private address of the sending host that’s trying to get to the Internet, while the out-
side local address is the address of the destination host. The latter is usually a public address
(web address, mail server, etc) and is how the packet begins its journey.
After translation, the inside local address is then called the inside global address and the out-
side global address then becomes the name of the destination host. Check out Table 1.2, which
lists all this terminology for a clear picture of the various names used with NAT.
TABLE 1.2 NAT Terms
Names Meaning
Local addresses Name of local hosts before translation
Global addresses Names of address after translation
Inside local Name of inside source address before translation
How NAT Works 5
TABLE 1.2 NAT Terms (continued)
Names Meaning
Outside local Name of destination host before translation
Inside global Name of inside hosts after translation
Outside global Name of outside destination host after translation
How NAT Works
Okay—now it’s time to look at how this whole NAT thing works. I’m going to start by using
Figure 1.2 to describe the basic translation of NAT:
In the example shown in Figure 1.2, host 10.1.1.1 sends an outbound packet to the border
router configured with NAT. The router identifies the IP address as an inside local IP address
destined to an outside network and translates the address and documents the translation in the
NAT table.
FIGURE 1.2 Basic NAT translation
Inside
10.1.1.3 3 DA
170.168.2.2
DA SA Host B
10.1.1.1 170.168.2.2 63.40.7.3
Internet
10.1.1.2
NAT Table
SA
10.1.1.1 Inside Local Inside Global
IP Address IP Address
10.1.1.3 170.168.2.4
10.1.1.1
10.1.1.2 170.168.2.3
10.1.1.1 170.168.2.2
6 Network Address Translation (NAT)
The packet is sent to the outside interface with the new translated source address. The external
host returns the packet to the destination host and the NAT router translates the inside global IP
address back to the inside local IP address using the NAT table. This is as simple as it gets.
Let’s take a look at a more complex configuration using overloading, or what is also referred
to as Port Address Translation (PAT). I’ll use Figure 1.3 to demonstrate how PAT works.
With overloading, all inside hosts get translated to one single IP address…hence the term
overloading. Again, the reason we have not run out of available IP addresses on the Internet is
because of overloading (PAT).
Take a look at the NAT table in Figure 1.3 again. In addition to the inside local IP address
and outside global IP address, we now have port numbers. These port numbers help the router
identify which host should receive the return traffic.
Port numbers are used at the Transport layer to identify the local host, in this example. If
we had to use IP addresses to identify the source hosts, that would be called static NAT and we
would run out of addresses. PAT allows us to use the Transport layer to identify the hosts,
which in turn, allows us to use (theoretically) up to 65,000 hosts with one real IP address.
FIGURE 1.3 NAT overloading example (PAT)
Inside
10.1.1.3
DA
170.168.2.2
DA SA Host B
10.1.1.1 170.168.2.2 63.40.7.3
Internet
DA
10.1.1.2 170.168.2.2
SA
10.1.1.1 Host C
63.40.7.3
10.1.1.1
NAT Table
Protocol Inside Local IP Inside Global IP Outside Global IP
10.1.1.1 Address: Port Address: Port Address: Port
TCP 10.1.1.3:1723 170.168.2.2:1492 63.41.7.3:23
TCP 10.1.1.2:1723 170.168.2.2:1723 63.41.7.3:23
TCP 10.1.1.1:1024 170.168.2.2:1024 63.40.7.3:23
How NAT Works 7
Static NAT Configuration
Let’s take a look at a simple basic static NAT configuration.
ip nat inside source static 10.1.1.1 170.46.2.1
!
interface Ethernet0
ip address 10.1.1.10 255.255.255.0
ip nat inside
!
interface Serial0
ip address 172.46.2.1 255.255.255.0
ip nat outside
!
In the above router output, the ip nat inside source command identifies what IP
addresses will be translated. In this configuration example, the ip nat inside source com-
mand configures a static translation between the inside local IP address 10.1.1.1 to the outside
global IP address 170.46.2.2.
If we look further down in the configuration, we see that we have an ip nat command under
each interface. The ip nat inside command identifies that interface as an inside source. The
ip nat outside command identifies that interface as an outside source.
Dynamic NAT Configuration
Dynamic NAT means that we have a pool of addresses that we will use to provide real IP
addresses to a group of users on the inside. We do not use port numbers, so we have to have real
IP addresses for every user trying to get outside the local network.
Here is a sample output of a dynamic NAT configuration:
ip nat pool dyn-nat 170.168.2.2 170.168.2.254
netmask 255.255.255.0
ip nat inside source list 1 pool dyn-nat
!
interface Ethernet0
ip address 10.1.1.10 255.255.255.0
ip nat inside
!
interface Serial0
ip address 170.168.2.1 255.255.255.0
ip nat outside
!
access-list 1 permit 10.1.1.0 0.0.0.255
!
8 Network Address Translation (NAT)
The ip nat inside source list 1 pool dyn-nat command tells the router to translate
IP addresses that match access-list 1 to an address found in the IP NAT pool named dyn-nat.
The ip nat pool dyn-nat 170.168.2.2 192.168.2.254 command creates a pool of
addresses that will be distributed to those hosts that require NAT.
PAT (Overloading) Configuration
This last example shows how to configure inside global address overloading. This is the typical
NAT that we would use today. It is rare that we would use static or dynamic NAT unless we
were statically mapping a server, for example.
Here is a sample output of a PAT configuration:
ip nat pool globalnet 170.168.2.1 170.168.2.1
netmask 255.255.255.0
ip nat inside source list 1 pool globalnet overload
!
interface Ethernet0/0
ip address 10.1.1.10 255.255.255.0
ip nat inside
!
interface Serial0/0
ip address 170.168.2.1 255.255.255.0
ip nat outside
!
access-list 1 permit 10.1.1.0 0.0.0.255
The nice thing about PAT is that the only difference with this configuration and the previous
dynamic NAT configuration is that our pool of addresses has shrunk to only one IP address, and
at the end of our ip nat inside source command we included the overload command.
Verifying NAT
Once you have configured the type of NAT you are going to use, typically overload (PAT), you
need to be able to verify the configuration.
To see basic IP address translation information, use the following command:
Router#show ip nat translation
When looking at the IP NAT translations, you may see many translations from the same host
to the same host at the destination. This is typical of many connections to the Web.
Testing Your Understanding 9
In addition, you can verify your NAT configuration with the debug ip nat command. This out-
put will show the sending address, the translation, and the destination address on each debug line.
Router#debug ip nat
How do you clear your NAT entries from the translation table? Use the clear ip nat
translation command. To clear all entries from the NAT table, use the asterisks (*).
Testing Your Understanding
Before we move on to the hands-on lab section of this update, let’s go through a couple NAT
examples and see if you can determine the configuration that needs to be used.
Look at Figure 1.4. Where would you implement NAT in this design, and what type of NAT
would you configure?
FIGURE 1.4 NAT example
Corporate Engineering Sales
Internet
In Figure 1.4, the NAT configuration would be placed on the corporate router and the con-
figuration would be dynamic NAT with overload (PAT).
In the next NAT example, what type of NAT is being used?
Ip nat pool todd-nat 170.168.10.10 170.168.10.20 netmask 255.255.255.0
The above command uses dynamic NAT. The pool in the command gives the answer away.
In the next NAT example, we’ll use Figure 1.5 to see if we can figure out the configura-
tion needed.
10 Network Address Translation (NAT)
FIGURE 1.5 Another NAT example
192.1.2.110 Lab_A
ISP S0/1
S0/0 F0/0
192.1.2.109 192.168.10.126
Console
The example in Figure 1.5 shows a border router that needs to be configured with NAT that
will allow six public IP address using 198.18.131.65-70. On the inside network, you have 63
hosts that use the private addresses of 192.168.10.129 through 190. What would your NAT
configuration be on the border router?
ip nat pool Todd 198.18.131.65 198.18.131.70 netmask 255.255.255.248
access-list 1 permit 192.168.10.128 0.0.0.63
ip nat inside source list 1 pool Todd overload
The command ip nat pool Todd 198.18.131.65 198.18.131.70 netmask 255.255
.255.248 sets the pool name as Todd and creates a dynamic pool of addresses for the NAT
to use from 65 to 70. Instead of the netmask command, you can also use the prefix-
length 29 statement. No, you cannot do this on router interfaces as well—I knew what you
were thinking!
If you do not understand the second line where the access-list is set, please see Chapter 10,
“Managing Traffic with Access Lists” of the CCNA Study Guide.
The command ip nat inside source list 1 pool Todd overload command sets the
dynamic pool to use Port Address Translation (PAT) by using the overload command.
Be sure to add the ip nat inside and ip nat outside statements on the appropriate
interfaces.
Hands-on Lab: Network Address
Translation
In this lab, you will configure NAT on router Lab_A to translate the private IP address of
192.168.10.0 to a public address of 171.16.10.0. I use three 2500 routers and one 2600 series
router, but you can use any type of routers for this lab. (The 2600 series router is the Lab_A router)
Table 1.3 shows the commands we will use and the purpose of each command.
Hands-on Lab: Network Address Translation 11
TABLE 1.3 Command Summary for NAT/PAT Hands-on Lab
Command Purpose
ip nat inside source list acl pool name Translates IPs that match the ACL from
the Pool
Ip nat inside source static Statically maps an inside address to an
inside_addr outside_addr outside address
IP nat pool name Creates an address Pool
IP nat inside Set an interface to be an inside interface
IP nat outside Set an interface to be an outside interface
Show ip nat translations Shows current NAT translations
R3
E0
E0
R2
S0
S0/0
R1
S0/2
S0/0
ISP
12 Network Address Translation (NAT)
Step 1: Preparing for NAT
In this step, you’ll setup your routers with IP addresses and RIP routing.
1. Configure the routers with the IP addresses listed here:
Router Interface IP Address
ISP S0 171.16.10.1/24
Lab_A S0/2 171.16.10.2/24
Lab_A S0/0 192.168.20.1/24
Lab_B S0 192.168.20.2/24
Lab_B E0 192.168.30.1/24
Lab_C E0 192.168.30.2/24
2. After you configure the routers, you should be able to ping from router to router, but since
we do not have a routing protocol running until the next step, you can only verify from one
router to another, but not through the network until RIP is set up. You can use any routing
protocol you wish, I am just using RIP for simplicity sake. Let’s just use a simple routing pro-
tocol to get this up and running.
3. On Lab_A, configure RIP routing, set a passive interface and configure the default network.
Lab_A#config t
Lab_A(config-router)#network 192.168.20.0
Lab_A(config-router)#network 171.16.0.0
Lab_A(config-router)#passive-interface s0/2
Lab_A(config-router)#exit
Lab_A(config)#ip default-network 171.16.10.1
The passive-interface command stops RIP updates from being sent to the ISP and the
ip default-network command advertises a default network to the other routers so they
know how to get the Internet.
4. On Lab_B, configure RIP routing
Lab_B#config t
Lab_B(config)#router rip
Lab_B(config-router)#network 192.168.30.0
Lab_B(config-router)#network 192.168.20.0
5. On Lab_C, configure RIP routing, but also use the passive-interface command since
there is no reason to send our routing table to the ISP.
Lab_C#config t
Lab_C(config)#router rip
Lab_C(config-router)#network 192.168.30.0
Hands-on Lab: Network Address Translation 13
6. On the ISP Router, configure a default route to the corporate network
ISP#config t
ISP(config)#ip route 0.0.0.0 0.0.0.0 s0
7. Configure the ISP router so you can telnet into the router without being prompted for
a password
ISP#config t
ISP(config)#line vty 0 4
ISP(config-line)#no login
8. Verify that you can ping from the ISP router to the Lab_C router and from the Lab_C
router to the ISP router. If you cannot, troubleshoot your network.
Step 2: Configuring Dynamic NAT
In this step, you’ll configure dynamic NAT on the Lab_A router.
1. Create a pool of address called GlobalNet on the Lab_A router. The pool should contain
a range of addresses of 171.16.10.50 through 171.16.10.55.
Lab_A(config)#ip nat pool GlobalNet 171.16.10.50 171.16.10.55 net
255.255.255.0
2. Create access-list 1. This list permits traffic from the 192.168.20.0 and 192.168.30.0 net-
work to be translated.
Lab_A(config)#access-list 1 permit 192.168.20.0 0.0.0.255
Lab_A(config)#access-list 1 permit 192.168.30.0 0.0.0.255
3. Map the access list to the pool that was created.
Lab_A(config)#ip nat inside source list 1 pool GlobalNet
4. Configure serial 0/0 as an inside NAT interface.
Lab_A(config)#int s0/0
Lab_A(config-if)#ip nat inside
5. Configure serial 0/2 as an outside NAT interface.
Lab_A(config-if)#int s0/2
Lab_A(config-if)#ip nat outside
6. Log in to the Lab_C router. Telnet from the Lab_C router to the ISP router.
Lab_C#telnet 171.16.10.1
7. Log in to the Lab_B router. Telnet from the Lab_B router to the ISP router.
Lab_B#telnet 171.16.10.1
14 Network Address Translation (NAT)
8. Execute the command show users from the ISP router. (This shows who is accessing the
VTY lines)
ISP#show users
What does it show as your source IP Address?________________
What is your Real Source IP address?__________________
The show users output should look something like this:
ISP>sh users
Line User Host(s) Idle Location
0 con 0 idle 00:03:32
2 vty 0 idle 00:01:33 171.16.10.50
* 3 vty 1 idle 00:00:09 171.16.10.51
Interface User Mode Idle Peer Address
ISP>
Notice that there is a one-to-one translation. Which means you must have a
real IP address for every host that wants to get to the Internet, which is not
always possible.
9. Leave the session open on the Core and connect to Lab_A. (use the Ctrl, Shift, 6, let go and
then press X).
10. Log in to your Lab_A router and view your current translations by entering the show ip
nat translation command. You should see something like this:
Lab_A#sh ip nat translations
Pro Inside global Inside local Outside local Outside global
--- 171.16.10.50 192.168.30.2 --- ---
--- 171.16.10.51 192.168.20.2 --- ---
Lab_A#
Oh my gosh, this really works!
11. If you turn on debug ip nat on the Lab_A router and then ping through the router, you
will see the actual NAT process take place, which will look something like this:
00:32:47: NAT*: s=192.168.30.2->171.16.10.50, d=171.16.10.1 [5]
00:32:47: NAT*: s=171.16.10.1, d=171.16.10.50->192.168.30.2
Hands-on Lab: Network Address Translation 15
Step 3: Configuring PAT
In this step, you’ll configure Port Address Translation (PAT) on the Lab_A router. We will use
PAT because we don’t want a one-to-one translation, which want to just use one IP address for
every user on the network.
1. On the Lab_A router, delete the translation table and remove the dynamic NAT pool
Lab_A#clear ip nat translation *
Lab_A#config t
Lab_A(config)#no ip nat pool GlobalNet 171.16.10.50 171.16.10.55 netmask
255.255.255.0
Lab_A(config)#no ip nat inside source list 1 pool GlobalNet
2. On the Lab_A router, create a NAT pool with one address called Lammle. The pool should
contain a single address 171.16.10.100 Enter the command below:
Lab_A#config t
Lab_A(config)#ip nat pool Lammle 171.16.10.l00 171.16.10.100 net 255.255.255.0
3. Create access-list 2. It should permit networks 192.168.20.0 and 192.168.30.0 to
be translated.
Lab_A(config)#access-list 2 permit 192.168.20.0 0.0.0.255
Lab_A(config)#access-list 2 permit 192.168.30.0 0.0.0.255
4. Map the access-list 2 to the new pool, allowing PAT to occur by using the overload
command.
Lab_A(config)#ip nat inside source list 2 pool Lammle overload
5. Login to the Lab_C router and telnet to the ISP router; Also, login to the Lab_B router and
telnet to the ISP router.
6. From the ISP router use the show users command. The output should look like this:
ISP>sh users
Line User Host(s) Idle Location
* 0 con 0 idle 00:00:00
2 vty 0 idle 00:00:39 171.16.10.51
4 vty 2 idle 00:00:37 171.16.10.50
Interface User Mode Idle Peer Address
ISP>
16 Network Address Translation (NAT)
7. From the Lab_A router use the show ip nat translations command.
Lab_A#sh ip nat translations
Pro Inside global Inside local Outside local Outside global
tcp 171.16.10.100:11001 192.168.20.2:11001 171.16.10.1:23 171.16.10.1:23
tcp 171.16.10.100:11002 192.168.30.2:11002 171.16.10.1:23 171.16.10.1:23
tcp 171.16.10.100:1024 192.168.20.2:11002 171.16.10.1:23 171.16.10.1:23
8. Also make sure that the debug ip nat command is on the Lab_A router. If you ping from
the Lab_C router to the ISP router, the output will look like this:
01:12:36: NAT: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [35]
01:12:36: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [35]
01:12:36: NAT*: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [36]
01:12:36: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [36]
01:12:36: NAT*: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [37]
01:12:36: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [37]
01:12:36: NAT*: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [38]
01:12:36: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [38]
01:12:37: NAT*: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [39]
01:12:37: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [39]