1. NAT / PAT

2. NAT / PAT Topics
Identify how NAT and PAT solve the limited IP address problem
Describe NAT and PAT operation
Configure NAT and PAT
Verify NAT and PAT
Purpose: This figure states the chapter objectives.
Emphasize: Read or state each objective so each student has a clear understanding of the chapter objectives.

3. Why Use NAT?
Inside
Outside SA SA
Internet
NAT border router
Use NAT if:
You need to connect to the Internet and your hosts do not have globally unique IP addresses
Purpose: This figure describes circumstances when you would implement NAT.
Emphasize: Examples when NAT may be employed include two companies that have duplicate internal addressing schemes merge, or a company changes its Internet Service Provider (ISP) but does not want to change its internal address scheme.
Transition: Advantages and disadvantages of using NAT follow.

4. NAT Implementation Considerations
Advantages
Conserves legally registered addresses Reduces address overlap occurrence
Increases flexibility when connecting to Internet
Eliminates address renumbering as network changes
Disadvantages
Translation introduces switching path delays Loss of end-to-end IP traceability
Certain applications will not function with NAT enabled
Purpose: This figure describes advantages and disadvantages of implementing NAT.
Note: The most obvious advantage is that NAT conserves the legally registered address scheme.
Transition: An overview of NAT follows.

5. NAT Overview and Terminology
Inside DA B
Host B DA SA C
Internet SA A
Simple NAT table D
Purpose: This figure is a transition into the NAT overview section. It also highlights some important NAT terms.
Emphasize: Highlight the different sending addresses on the packet before it enters the router and after it leaves the router. Compare those addresses to those listed on the NAT table.
Describe each term as it relates to the figure.
Note: The letters on the figure correspond to the descriptions in the text. Descriptions for outside local IP address and extended translation entry are not represented graphically.
Easy IP is a related feature to NAT available on Cisco routers. Configuring Easy IP is not taught in this course. The Easy IP (Phase 1) feature combines NAT and Point-to-point (PPP)/Internet Protocol Control Protocol (IPCP). This feature enables a Cisco router to automatically negotiate its own registered WAN interface IP address from a central server and enable all remote hosts to access the global Internet using this single registered IP address. Because Easy IP (Phase 1) uses existing port-level multiplexed NAT functionality within the Cisco IOS software, IP addresses on the remote LAN are invisible to the Internet.
Reference: For a complete description of the Easy IP configuration commands, refer to the “Easy IP Commands” chapter in the Dial Solutions Command Reference.
Inside Local IP
Inside Global
Address
IP Address A B

6. NAT Operation NAT functions: Translation inside local addresses
Internet
NAT functions:
Translation inside local addresses
Overloading inside global addresses
Purpose: This figure is a transition that highlights the NAT functions that are presented in the next few figures.
Emphasize: The next few figures discuss the following NAT functions:
Translating inside global addresses
Overloading inside global addresses
Handling overlapping networks
Transmission Control Protocol (TCP) load distribution
Transition: The next figure describes translating inside global addresses.
NAT table
Inside Local
Inside Global
IP Address
IP Address

7. Translating Inside Local Addresses4 DA 5 3
Host B DA SA
Internet
Purpose: This figure explains how address translation works.
Emphasize: Later when students learn to configure address translation, they will be able to use either static NAT configuration or dynamic NAT configuration.
Transition: The next figure describes overloading inside global addresses. SA 1 2
NAT table
Inside Local IP Address
Inside Global
IP Address

8. Overloading Inside Global Addresses4 DA
Host B 5 3 DA SA 4
Internet DA
Host C 1 2
NAT table
Purpose: This figure explains how overloading inside global addresses works.
Emphasize: Overloading inside global address translation is Port Address Translation (PAT). How to configure PAT on a Cisco 700 series router is described later in this chapter.
Transition: The next figure describes TCP load distribution. SA
Protocol
Inside Local IP
Inside Global IP
Outside Global
Address: Port
Address: Port
IP Address: Port
TCP
:1723
:1492
:23
TCP
:1723
:1723
:23
TCP
:1024
:1024
:23

9. Static NAT Configuration Example
ip nat inside source static !
interface Ethernet0
ip address
ip nat inside
interface Serial0
ip address
ip nat outside
This interface connected to the inside network.
This interface connected to the outside world.
Purpose: This figure displays the static NAT configuration output.
Emphasize: Highlight the inside and outside interfaces on this configuration.
Note: This figure and the subsequent NAT configuration figures only display the configurations necessary to configure NAT translation. Other commands may be necessary for routing.
Maps the inside local address to the inside global address.

10. Dynamic NAT Configuration
ip nat pool dyn-nat netmask
ip nat inside source list 1 pool dyn-nat !
interface Ethernet0
ip address
ip nat inside
interface Serial0
ip address
ip nat outside
access-list 1 permit
This interface connected to the inside network.
Purpose: This figure displays the dynamic NAT configuration output.
This interface connected to the outside world.
Translate between inside hosts addressed from /24 to the globally unique /24 network.

11. Configuring Inside Global Address Overloading
ip nat pool ovrld-nat
netmask
ip nat inside source list 1 pool ovrld-nat overload !
interface Ethernet0/0
ip address
ip nat inside
interface Serial0/0
ip address
ip nat outside
access-list 1 permit
Purpose: This figure describes how to configure inside global address overloading.

12. Verifying NAT Basic IP address translation Router#show ip nat trans
Pro Inside global Inside local Outside local Outside global
IP address translation with overloading
Router#sh ip nat trans
Pro Inside global Inside local Outside local Outside global
tcp : : : :23
tcp : : : :23
Purpose: This figure describes how to verify your NAT configuration output.
Emphasize: The upper output box displays the typical NAT table. The lower output box displays the NAT table with overloading.
Note: 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.
Unique TCP port numbers are used to distinguish between hosts.
A translation for a Telnet is still active. Two different inside hosts appear on the outside with a single IP address.

13. Troubleshooting NAT An example address translation inside-to-outside.
Router#debug ip nat
NAT: s= > , d= [0]
NAT: s= , d= > [0]
NAT: s= > , d= [1]
NAT: s= > , d= [2]
NAT: s= > , d= [3]
NAT*: s= , d= > [1]
NAT: s= , d= > [1]
NAT: s= > , d= [4]
NAT: s= > , d= [5]
NAT: s= > , d= [6]
NAT*: s= , d= > [2]
Purpose: This figure describes the debug ip nat command.
Emphasize: Show the sending address, the translation, and the destination address on each debug line.
An example address translation inside-to-outside.
A reply to the packet sent.
An example TCP conversation, inside-to-outside.
* Indicates translation was in the fast path.

14. Clearing NAT Translation Entries
Router#sh ip nat trans
Pro Inside global Inside local Outside local Outside global
tcp : : : :23
tcp : : : :23
router#clear ip nat trans *
router#
router#show ip nat trans
All entries are cleared.
router#show ip nat trans
Pro Inside global Inside local Outside local Outside global
udp : : : :53
tcp : : : :23
tcp : : : :23
router#clear ip nat trans udp inside
tcp : : : :23
tcp : : : :23
Purpose: This figure describes how to clear your NAT entries from the translation table.
Emphasize: The “*” clears all entries from the NAT table.
Both output boxes in the figure show how the NAT table looks before and after translations are cleared.
is cleared.

15. NAT Configuration Lab.pdf NAT Configuration Lab.pkt
Lab file
NAT Configuration Lab.pdf
Lab scenario
NAT Configuration Lab.pkt