Home > Articles > Cisco Certification > CCNP Security / CCSP > Basic NAT Concepts and Configuration

Basic NAT Concepts and Configuration

  • Article is provided courtesy of Cisco Press.
  • Date: Jul 6, 2011.

Contents

  1. Basic NAT Concepts and Configuration

Article Description

The use of Network Address Translation (NAT) has been wide spread for a number of years; this is because it is able to solve a number of problems with the same relatively simple configuration. At its most basic, NAT enables the ability to translate one set of addresses to another. This enables traffic coming from a specific host to appear as though it is coming from another and do it transparently. This article looks at some of the basic concepts that are used when configuring NAT and reviews the configuration steps required to get NAT working.

Like this article? We recommend

CCNP Security Secure 642-637 Official Cert Guide

CCNP Security Secure 642-637 Official Cert Guide

$69.99

The use of Network Address Translation (NAT) has been widespread for a number of years; this is because it is able to solve a number of problems with the same relatively simple configuration. At its most basic, NAT enables the ability to translate one set of addresses to another; this enables traffic coming from a specific host to appear as though it is coming from another and do it transparently. This article looks at some of the basic concepts that are used when configuring NAT and reviews the configuration steps required to get NAT working.

NAT Concepts

There are a number of different concepts that must be explained in order to really get a good understanding of how NAT operates, which ultimately makes the configuration of NAT increasingly simple. This section reviews these different concepts and begins with an understanding of how NAT can be used. Some of the main uses for NAT include:

  • Translation of non-unique addresses into unique addresses when accessing the Internet:
  • This is one of the most common uses of NAT today; almost every household that has a “router” to access the Internet is using NAT on this device to translate between internal private address and public Internet addresses.

  • Translation of addresses when transitioning internal addresses from one address range into another (this is common when the organization of addresses within a company is being changed):
  • This is often done when a company is transitioning their IP addressing plan; common scenarios include when expanding (and the IP addressing plan was not built sufficiently when the initial addresses were assigned) and when a company is merging with another with potential overlapping addresses.

  • When simple TCP load sharing is required across many IP hosts:
  • This is very common, as many highly used servers are not really a single machine but a bank of several machines that utilize load balancing. In this scenario, commonly, a single public address is translated into one of several internal addresses in a round robin fashion.

This is not a complete list of every possible way that NAT can be configured but simply a list of the most common ways that it is used in modern networks.

There are a couple of main concepts that also must be reviewed and understood before configuring NAT:

  • Inside and Outside Addresses
  • NAT types

Inside and Outside Addresses

In typical NAT configurations, interfaces are placed into one of two categories (or locations): inside or outside. Inside indicates traffic that is coming from within the organizational network. Outside indicates traffic that is coming from an external network that is outside the organizational network.

These different categories are then used to define different types of address depending on location of the address and how it is being “seen”. These different types include:

  • inside local address: This is the inside address as it is seen and used within the organizational network.
  • inside global address: This is the inside address as it is seen and used on the outside of the organizational network.
  • outside local address: This is the outside address as it seen and used within the organizational network.
  • outside global address: This is the outside address as it is seen and used on the outside of the organizational network.

NAT Types

Another important concept to be familiar with is the different types of NAT and how they are defined. On most networks there are three different types of NAT that are defined:

  • Static address translation (Static NAT): This type of NAT is used when a single inside address needs to be translated to a single outside address or vice versa.
  • Dynamic address translation (Dynamic NAT): This type of NAT is used when an inside address (or addresses) need to be translated to an outside pool of addresses or vice versa.
  • Overloading (Port Address Translation (PAT): This type of NAT is a variation on dynamic NAT. With dynamic NAT, there is always a one to one relationship between inside and outside addresses; if the outside address pool is ever exhausted, traffic from the next addresses requesting translation will be dropped. With overloading, instead of a one to one relationship, traffic is translated and given a specific outside port number to communicate with; in this situation, many internal hosts can be using the same outside address whil utilizing different port numbers.

NAT Configuration

There are few methods (and commands) that are used to configure NAT. The main three methods include one for static NAT, one for Dynamic NAT, and one for TCP load sharing.

Static NAT Configuration

There a few steps that are required when configuring static NAT; the number of the commands depends on whether there will be more than one static translation:

1

Enter global configuration mode.

router#configure terminal

2

Configure the static NAT translation (this command can be used multiple times depending on the number of static translations required). The overload keyword enables the use of PAT.

router(config)#ip nat inside source static local-ip global-ip [overload]

3

Enter interface configuration mode for the inside interface.

router(config)#interface interface-id

4

Configure the interface as the inside NAT interface.

router(config-if)#ip nat inside

5

Enter interface configuration mode for the outside interface.

router(config-if)#interface interface-id

6

Configure the interface as the outside NAT interface.

router(config-if)#ip nat outside

7

Exit configuration mode.

router(config-if)#end

Static NAT Configuration Example

To ensure the configuration of static NAT is clear, let’s look at an example (Figure 1) that explains the concepts described above:

In this example, the inside host 192.168.1.20 will be translated to 172.16.1.5 when sending traffic out of interface f0/1.

Dynamic NAT Configuration

Dynamic NAT requires a few additional commands over a static configuration as the source of the traffic and the NAT address pool must be configured:

1

Enter global configuration mode.

router#configure terminal

2

Configure the dynamic NAT address pool.

router(config)#ip nat pool pool-name start-ip end-ip {netmask netmask | prefix-length prefix-length}

3

Configure a static access list to define the addresses to be translated.

router(config)#access-list access-list-number permit source [source-wildcard]

4

Configure the dynamic NAT translation. The overload keyword enables the use of PAT.

router(config)#ip nat inside source list access-list-number pool pool-name [overload]

5

Enter interface configuration mode for the inside interface.

router(config)#interface interface-id

6

Configure the interface as the inside NAT interface.

router(config-if)#ip nat inside

7

Enter interface configuration mode for the outside interface.

router(config-if)#interface interface-id

8

Configure the interface as the outside NAT interface.

router(config-if)#ip nat outside

9

Exit configuration mode.

router(config-if)#end

Dynamic NAT Configuration Example

To ensure the configuration of dynamic NAT is clear, let’s look at an example (Figure 2) that explains the concepts described above:

In this example, the hosts that have addresses from 192.168.1.1 through 192.168.1.254 will be translated to an address from the pool which includes addresses from 172.16.1.10 through 172.16.1.20; if a 12th host attempts to send traffic out of the f0/1 interface, the translation will fail.

TCP Load Balancing Configuration

The TCP load balancing feature enables the ability to assign a single outside address that is translated into one of a pool of addresses in order to balance the load of traffic over a number of different hosts. The following commands are used to configure TCP load balancing:

1

Enter global configuration mode.

router#configure terminal

2

Configure the NAT address pool that contains the list of real host IP addresses to load balance to.

router(config)#ip nat pool pool-name start-ip end-ip {netmask netmask | prefix-length prefix-length} type rotary

3

Configure a static access list to define the virtual address that will be used for outside communication.

router(config)#access-list access-list-number permit source [source-wildcard]

4

Configure TCP server load balancing.

router(config)#ip nat inside destination-list access-list-number pool pool-name

5

Enter interface configuration mode for the inside interface.

router(config)#interface interface-id

6

Configure the interface as the inside NAT interface.

router(config-if)#ip nat inside

7

Enter interface configuration mode for the outside interface.

router(config-if)#interface interface-id

8

Configure the interface as the outside NAT interface.

router(config-if)#ip nat outside

9

Exit configuration mode.

router(config-if)#end

TCP Load Balancing Configuration Example

To ensure the configuration of TCP load balancing is clear, let’s look at an example (Figure 3) that explains the concepts described above:

In this example, all traffic that is addressed to 192.168.1.5 will be translated and sent to the hosts with addresses from 192.168.1.10 through 192.168.1.20 in a round robin fashion.

Summary

There are certainly many different situations where the functionality of NAT can be used. This article takes a look at some of the ways that NAT can be configured and offers examples of how the functionality can be implemented. Hopefully this article has made the concepts and configuration of NAT a little simpler to understand so that they can be implemented with little trouble or confusion.

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.

Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children

This site is not directed to children under the age of 13.

Marketing

Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020