Home > Articles > Security > Network Security

Network Address Translation

This chapter discusses the topic of Network Address Translation (NAT): what it is, why it was created, and how you can implement it in FireWall-1. This sample chapter was excerpted from Essential Checkpoint Firewall-1.
This chapter is from the book

This chapter discusses the topic of Network Address Translation (NAT): what it is, why it was created, and how you can implement it in FireWall-1. I first discuss the reasons NAT was created and how NAT is implemented in FireWall-1. Next, I show a step-by-step example of how to implement NAT in a network. I then talk about some of the inherent limitations of NAT and discuss a couple of ways to work around these limitations. Finally, I talk about troubleshooting NAT with a packet sniffer.

By the end of this chapter, you should be able to:

  • Understand why NAT is necessary
  • Identify what NAT actually does
  • Identify why NAT does not always work
  • Effectively troubleshoot NAT problems with a packet sniffer
  • Implement a NAT Configuration

Introduction

Back in the old days of the Internet, the TCP/IP address space (as defined by IPv4, the version of IP used today) was thought to be more than enough. Organizations could reserve their own address space through the Internet Assigned Numbers Authority (IANA, now called the Internet Corporation for Assigned Names and Numbers [ICANN]), and anyone who wanted a block of IP addresses generally got them.

Since the early 1990s, various people have been predicting that the IPv4 address space will simply run out of available addresses. This is partially due to the explosive growth of the Internet, but it is also due to how the IPv4 address space is divided. Many organizations that were allocated address space early on simply have more address space allocated to them than they are using on the Internet. There are also parts of the IPv4 address space that are not legal for hosts to be assigned to on the Internet, namely the multicast (224.0.0.0/240.0.0.0 mask) and the Class E (240.0.0.0/240.0.0.0) address spaces.

As organizations are connecting to the Internet, some are discovering that their internal network does not connect well to the Internet. The main reason for this is usually a conflict in addressing. Long before the Internet was a household word, some corporations set up their internal networks using made-up addresses. However, you cannot simply make up addresses and use them on the Internet. You must use IP addresses assigned by IANA or an ISP. Renumbering a large, internal network would be a daunting task, not to mention that your ISP or IANA is not likely to give you enough addresses to cover all your hosts. Then again, does every host on your internal network really need to be uniquely addressable on the Internet?

IPv6 (the next version of IP) has far more address space—128 bits of address space versus the 32 provided by IPv4—which will solve this problem. However, most of today's Internet is still running IPv4 and probably will be for some time to come. A solution is needed that will help extend the IPv4 address space that is used today.

Network Address Translation does exactly this. It is a technology that allows hosts to transparently talk to one another with addresses that are agreeable to each other. To put it another way, it allows hosts with illegal or private address space to talk with hosts on a public network and vice versa. It is a godsend for network managers who have limited address space or want to make better use of the address space they have without having to subnet, thus reducing the number of IPs that can be used. NAT can also be perceived as a security enhancement because a firewall is required for communication between the hosts. NAT, as it is commonly implemented today, is described in RFC3022.1

NAT is implemented as part of the FireWall-1 Kernel Module that sits between the data link and network layers. As such, NAT can be provided transparently without the client's or the server's knowledge. Application proxies, by their nature, can also provide this functionality, as they originate all connections coming from the internal network. However, proxies usually are not transparent and do not usually give you the level of control you have over FireWall-1's NAT functionality. You can modify the source, destination, and service port of any connection going through FireWall-1.

Consider the following example (see Figure 9-1). Let's say your ISP gives you a /29 block of addresses (net mask 255.255.255.248). If you were to use this address space between your Internet router and your firewall, the address space would break down into the host numbers listed in Table 9-1.2

Table 9-1 Breakdown of 192.168.0.0/29 address space

Host Number

Description

.0

Network identifier (cannot be used by hosts)

.1

Internet router

.2

Firewall

.3

Available

.4

Available

.5

Available

.6

Available

.7

Broadcast address (cannot be used by hosts)


Between the broadcast address, the network address, your firewall, and your Internet router, you have a grand total of four usable IP addresses. With NAT, you can:

  • Give your e-mail, intranet Web server, and Web server externally reachable IP addresses

  • Allow all your clients to access the Internet using the firewall's external IP address

  • Have all of your computers protected by your firewall

  • Change ISPs without having to renumber your internal network

Figure 9-1 illustrates a sample network.

Figure 9-1 Sample network diagram

Although NAT does add an extra layer of protection and gives you flexibility, there are some downsides to NAT:

  • Using NAT is like using proxies in that NAT must be updated to handle new applications. As a result, it is not compatible with every application that exists today or in the future.

  • NAT requires additional work to maintain. This is discussed in more detail in the "Implementing NAT, a Step-by-Step Example" section later in this chapter.

  • Only so many connections can be hidden behind a single IP address.

  • NAT requires extra memory and CPU on the gateway. In most cases, this is negligible, but it starts becoming noticeable when over 20,000 connections through a single gateway are subject to NAT.

More information about the disadvantages of using NAT is documented in RFC3027.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT 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 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@informit.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 InformIT. 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.informit.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