IP Addresses, Packets and Routers
Lesson 9: Network Address Translation (NAT)

In this lesson, we'll understand what is meant by the term "protocol", and how that relates to the concept of a standard. We will also start considering how many functions have to be performed to interoperate systems, and how these functions might be organized.

This free online network training course lesson is in both the CTNS Certification Package and the CTA Certification Package.

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Certification Packages That Include This Course

CTNS Certification Package

Six online courses plus TCO Certification covering the core knowledge needed for telecommunications today:
  • The PSTN
  • Wireless Telecommunications
  • The OSI Layers and Protocol Stacks
  • Ethernet, LANs and VLANs
  • IP Networks, Routers and Addresses
  • MPLS and Carrier Networks

TCO Certification, Certificate and Letter of Reference.

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CTA Certification Package

Sixteen online courses covering telecom, datacom and networking for non‑engineers from A-Z, plus the prestigious TCO Certified Telecommunications Analyst certification.

Includes the six CTNS courses plus

  • The Telecommunications Industry
  • Digital
  • Transmission Systems and Fiber Optics
  • IP Security
  • The Internet, and many more

TCO Certification, Certificate and Letter of Reference.

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Study Guide Notes For This Lesson

These are the words that are displayed and spoken during the lesson. Get these notes for the whole course in the Certification Study Guide, available in print or eBook. Many people tell us a printed companion book enhances their learning!

In the previous lesson, we covered private IP addresses, and why these were preferable to use on an in-building network.

We also noted that if any of the users on the private network want to receive packets from the Internet, a public IP address is required.

The question we explore in this lesson is how to enable Internet communications for all users in-building without having to rent a public IP address for every user?

A solution is to use a Network Address Translator (NAT).

When a computer on the private side initiates communications with a server, it populates the source IP address field in the packet header with its private address and the destination IP address field with the public IP address of the server.

The packet is then transmitted in a MAC frame to the computer's "default gateway", which is the Customer Edge router. This device is performing the NAT function. The NAT changes the source IP address from the private IP address of the sender to the public IP address of the NAT, i.e. the CE router, then transmits the packet in a frame on the public network (the Internet).

The Internet server of course uses the source address in the packet it receives as the destination address to answer back to the client. Therefore, it will send the response back addressed to the NAT.

When the NAT receives the packet, it changes the destination IP address on the packet received from the Internet to the private IP address of the appropriate computer, then transmits the packet in a MAC frame to the computer.

How does the NAT know what computer on the private network a packet received from the Internet is intended for?

It turns out that the NAT uses the Layer 4 header to keep track of things.

The Layer 4 header (TCP or UDP) begins with two octets that are called the "source port" then two octets for the "destination port". These fields are used to indicate which application on a computer the message is being sent from and to.

The NAT selects an arbitrary "fake" port number to identify a computer on the private network, and records this port number against the private address in a table.

When a packet is transmitted to the Internet, the NAT records the actual source port number then changes the source port value to the "fake" port number.

When the reply from the server is received from the Internet, it has the "fake" port number in the destination port field of the Layer 4 header. The NAT uses this to look up the correct private IP address and correct port number and enter those values in the destination address and destination port number fields, thus relaying the incoming packet to the correct computer on the private network.

NAT provides a number of advantages

1. A NAT allows multiple computers in-building to share a single Internet address and Internet connection.

2. A NAT provide a truly "always-on" connection to the Internet. Services like DSL and Cable modem described as "always on" are always connected at the Physical Layer. They do not provide "always on" at the Network Layer, since DHCP must be run every time the attached device restarts to get a public IP address. When a NAT is inserted, it runs DHCP to get the public IP address; so if the NAT is not powered off, the site will always have a public IP address assigned, and thus a connection to the Internet always ready for immediate use.

3. A NAT shields machines from attacks from the Internet. Since a private IP address is not reachable from the Internet, there is no way for a machine on the Internet to initiate communications to a machine on the private network. The only device exposed to the Internet is the NAT. Normally, the NAT is not running on a computer running Windows, so attackers have a greatly diminished chance of finding an vulnerability to exploit compared to connecting a computer running Windows naked onto the Internet.

Devices that perform this function are available in industrial-strength versions from companies like Cisco.

Hardware devices to do this are also available for about $20 from companies like Linksys for use on a DSL or cable modem connection. They often include both an Ethernet switch and an 802.11 wireless LAN access point for the private network side.

Most ISPs now provide the CE router with NAT function integrated in a device that includes the DSL or Cable modem they supply.

Network Address Translation (NAT)Network Address Translation (NAT)

Learning Objectives - What You Will Learn

Upon completion of this course, you will be able to explain:
  • The concept of statistical multiplexing, also known as oversubscription, overbooking and bandwidth on demand, why and how it can be implemented and its benefits.
  • What a private network is
  • What a router is and how it implements the network by connecting data links
  • How routers move packets between broadcast domains, including VLANs
  • How routers also act as a point of control for traffic, called packet filtering
  • The basic structure and contents of a routing table
  • The Customer Edge
  • IPv4 address blocks: Class A, Class B and Class C, and dotted-decimal notation
  • Static addresses and dynamic addresses, and how and why DHCP is used to assign both
  • Public addresses and private addresses, how, why and where each is used
  • Network Address Translation for interfacing domains where public addresses are used with those where private addresses are used
  • The improvements and changes between IPv4 and IPv6, and
  • The types of IPv6 addresses, how IPv6 addresses are allocated to ISPs then assigned to users, and how each residence gets 18 billion billion IPv6 addresses.

Lessons In This Course

Lesson 1 is the Introduction to the Course.

Lesson 2. Review: Channelized Time-Division Multiplexing (TDM) We'll review the idea of channelized Time-Division Multiplexing, what channels are, and how they can be used to aggregate traffic onto a high-speed circuit. Then we'll raise some questions: is that an efficient way to connect devices that produce traffic in bursts, which means devices that are normally doing nothing? And what about the problem of a single point of failure for all the aggregated traffic? Subsequent lessons explore the answers to those questions.

Lesson 3. Statistical TDM: Bandwidth-on-Demand. In this lesson, we'll understand how circuits that move bits constantly can be used efficiently when the user's traffic profile is: "idle most of the time, interspersed with bursts of data every once in a while." The answer is overbooking. This is also called statistical multiplexing and bandwidth-on-demand, and is a key part of a packet network: the internal circuits are heavily overbooked, to give users the highest speed at the lowest cost. It is necessary to know the users' historical demand statistics – also called their traffic profile – to know how much to overbook, hence the term statistical multiplexing.

Lesson 4. Private Network: Bandwidth on Demand + Routing. The purpose of this lesson is to expand the discussion of the previous lesson to include multiple circuits. The result is called a private network, and is the simplest framework for understanding routers, routing, network addresses and bandwidth-on-demand.

Lesson 5. Routers In this lesson, we'll take a closer look at a router, more precisely identifying the functions a router performs to implement a packet network, and understand how a router routes by examining the basic structure and content of a routing table. We'll also understand how the router can act as a point of control, denying communications based on criteria including network address and port number, why this is implemented and its limitations. The term Customer Edge (CE) is defined in this lesson.

Lesson 6. IPv4 Addresses Here, we'll understand IPv4 addresses, address classes and the dotted-decimal notation used to represent them.

Lesson 7. DHCP In this lesson, we'll cover DHCP: the Dynamic Host Configuration Protocol, and understand the mechanism by which a machine is assigned an IP address. We'll also understand how the "dynamic" host configuration protocol can be used to assign static addresses to machines and the advantages of this method.

Lesson 8. Public and Private IPv4 Addresses The purpose of this lesson is to define the terms "public" and "private" IP address, review how IP addresses are assigned and the costs for those addresses, then cover the ranges of IPv4 addresses that are used as private addresses, and understand how and why they are used.

Lesson 9. Network Address Translation In this lesson, we'll explore how private IPv4 addresses used in-building and a public address required for Internet communications can be joined together with a software function called Network Address Translation. 

Lesson 10. IPv6 Overview Completing this course on IP, we'll first review the next generation of IP: IPv6, understand the improvements compared to IPv4 and review the format of the IPv6 packet and its header.

Lesson 11. IPv6 Address Allocations and Assignment Finally, we examine the structure of the 128-bit IPv6 address, review the different kinds of IP addresses, the organizations that allocate them, and the current plans for how addresses will be assigned to end users… and how every residence gets 18 billion billion IPv6 addresses.

CTNS Study Guide and Companion Reference Textbook

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Overview of Courses in the CTNS Certification Package

Like Teracom's famous core training Course 101 "Telecom, Datacom and Networking for Non-Engineers", our very popular core training DVD-Video packages and the Telecom 101 textbook, the Certified Telecommunications Network Specialist Certification Package begins with the Public Switched Telephone Network, then a course on Wireless Telecommunications, followed by four courses covering IP telecommunications and IP telecom networks.

If you are interested only in IP telecommunications, the CIPTS: Certified IP Telecom Network Specialist package may be appropriate, as it skips the traditional telephony and wireless and goes directly to the IP telecommunications courses.

If your goal is to build a full, rounded knowledge of telecommunications, then understanding the history, structure and operation of the telephone network built over the past 135 years or more is the starting point for everything else.

Loops and Trunks   •  POTS   •  Circuit-Switching   •  LECs, CLECs and IXCs   •  Analog   •  Voiceband   •  DTMF   •  SS7

We begin with a history lesson, understanding how and why telephone networks and the companies that provide them are organized into local access and inter-city transmission, or as we will see, Local Exchange Carriers (LECs) and Inter-Exchange Carriers (IXCs).

Then we will establish a basic model for the PSTN and understand its main components: Customer Premise, Central Office, loop, trunk, outside plant, circuit switching, attenuation, loop length, remotes, and why knowledge of the characteristics of the loop remains essential knowledge even though we are moving to Voice over IP.

Next, we'll cover aspects of telephony and Plain Ordinary Telephone Service, including analog, the voiceband, twisted pair, supervision and signaling including DTMF. The course is completed with an overview of SS7, the control system for the telephone network in the US and Canada.

On completion of this course, you will be able to draw a model of the Public Switched Telephone Network, identify and explain its components and technologies including:

  • Loops and trunks, CO, telephone switches and circuit-switching
  • Twisted pair, the outside plant, remotes, fiber to the neighborhood
  • The founding, breakup and re-emergence of AT&T in the US; Bell & TELUS in Canada
  • LECs, IXCs and CLECs
  • Plain Ordinary Telephone Service (POTS):
  • Analog, the voiceband, how it relates to copper wires, electricity, circuits and sound
  • Supervision, dial tone, ringing, lightning protection, tip and ring, -48 volts
  • Touch-tone and DTMF
  • Basics of SS7 and call routing
Mobility   •  Cellular Networks   •  Internet over Cellular   •  GSM   •  UMTS and CDMA   •  LTE   •  WiFi   •  Satellite

In many parts of the world, particularly outside Canada, the US and Western Europe, the physical telephone network is wireless, as deploying radio transceivers is far cheaper than embarking on a new project to pull copper wires and/or fiber to every residence.

Most of this course is devoted to mobile wireless telecommunications. We begin with basic concepts and terminology including base stations and transceivers, mobile switches and backhaul, handoffs, cellular radio concepts and digital radio concepts.

Then, we cover spectrum-sharing technologies and their variations in chronological order: GSM/TDMA vs. CDMA for second generation, 1X vs. UMTS CDMA for third generation along with their data-optimized 1XEV-DO and HSPA, how Steve Jobs ended the standards wars with the iPhone and explaining the OFDM spectrum-sharing method of LTE for 4G.

This course is completed with a lesson on WiFi, or more precisely, 802.11 wireless LANs, and a lesson on satellite communications.

You'll gain a solid understanding of the key principles of wireless and mobile networks:

  • Coverage, capacity and mobility
  • Why cellular radio systems are used
  • Mobile network components and operation
  • Registration and handoffs
  • Digital radio
  • "Data" over cellular: Internet access
  • Cellular technologies: FDMA, TDMA, CDMA, OFDM
    Generations: 1G, 2G, 3G, 4G
  • Systems: GSM, UMTS, 1X, HSPA, LTE
  • WiFi, 802.11 wireless LANs
  • Satellite communications

The remaining four courses in the CTNS package are on the "IP" telecommunications network and its three main enabling technologies: Ethernet, IP and MPLS, and beginning with the OSI model and its layers to establish a framework.

If you'd prefer to take just these four "IP" courses, check out the Certified IP Telecom Network Specialist package.

Protocols & Standards   •  OSI Model   •  Layers   •  Protocol Stacks   •  How Protocol Stacks Work

This course establishes a framework for all of the subsequent discussions: the OSI 7-Layer Reference Model, which identifies and divides the functions to be performed into groups called layers.

This framework is required to sort out the many functions that need to be performed, and to be able to discuss separate issues separately.

First, we'll define the term "protocol" and compare that to a standard. Then we'll define "layer" and how a layered architecture operates, and provide an overview of the name, purpose and function of each of the seven layers in the OSI model.

Then, we'll go back through the story more slowly, with one lesson for each of the layers, examining in greater detail the functions that have to be performed and giving examples of protocols and how and where they are used to implement particular layers.

The result is a protocol stack, one protocol on top of another on top of another to fulfill all of the required functions. To make this more understandable, this course ends with the famous FedEx Analogy illustrating the concepts using company-to-company communications, and an analogy of Babushka dolls to illustrate how the protocol headers are nested at the bits level.

On completion of this course, you will be able to:

  • Define a protocol and differentiate that from a standard
  • Explain why a layered architecture is required
  • List the seven layers of the OSI model, the name, purpose and functions of each
  • Explain how the layers relate to each other
  • Explain how a protocol stack operates and protocol headers.
MAC Addresses   •  802.3 and Ethernet   •  Broadcast Domains   •  LAN Cables   •  LAN Switches   •  VLANs
This course is everything you need to know about LANs.  As we will see in the OSI Layers course, this course could also be titled "Layer 2".
We'll begin with the original LAN: Ethernet and its bus topology, defining "broadcast domain" and explaining its fundamental operation and characteristics: CSMA-CD access control, MAC addresses and MAC frames.
Then we'll cover the IEEE 802 standards and the evolution of Ethernet from 10BASE-T to Gig-E, LAN cables and the TIA-568 cable categories, basic cabling design; what "bridging" means and how a LAN switch works.
This course is completed with the important concept of VLANs: defining broadcast domains in software, a key part of basic network security practice.
On completion of this course, you will be able to explain
• Ethernet and the original bus design
• What a broadcast domain is
• What MAC addresses are
• The access control mechanism
• The IEEE 802 series of standards, 802.2 and 802.3
• Gigabit Ethernet on copper and fiber
• Codes like 100BASE-T
• LAN cables and the TIA-568 cable categories
• LAN switches, also called "Layer 2" switches
• How and why VLANs are used to separate devices
IP Packets   •  Packet Networks   •  Routers   •  Static, Dynamic, Public, Private Addresses   •  NAT   •  IPv6
This is a comprehensive course on IP addresses, routers and packets. Referring to the OSI Layers, this course could also be called Layer 3. We begin with the two basic principles of packet networks: bandwidth on demand, also known as overbooking or statistical multiplexing; and packet-switching, also known as packet forwarding or routing.
We'll understand what routers do and where they are located, routing tables and the basic operation of a router and the standard strategy deploying an edge router between the LANs and the WAN at each location.
Then we'll cover IP version 4: address classes and how they are assigned to Regional Internet Registries then ISPs then end-users, dotted-decimal notation, static addresses, dynamic addresses and DHCP, public addresses, private addresses and NAT.
The course concludes with IPv6: the IPv6 packet and changes from IPv4, IPv6 address allocations and assignments and end up understanding how IPv6 subnets will be assigned to broadcast domains and 18 billion billion addresses per residence.

On completion of this course, you will be able to explain:

  • What a packet is
  • What a router is
  • Overbooking and bandwidth on demand
  • Why and how it can be implemented
  • What a network is, what a private network is
  • How routers implement a network by connecting links
  • How routers move packets between broadcast domains
  • Basic network design and security: packet filtering
  • The basic structure and contents of a routing table
  • The Customer Edge
  • IPv4 address blocks: Class A, Class B and Class C
  • Dotted-decimal notation
  • Static addresses and dynamic addresses
  • DHCP and how and why it is used to assign both
  • Public addresses and private addresses
  • How, why and where each is used
  • NAT: Network Address Translation
  • IPv6
  • How IPv6 addresses are allocated to ISPs
  • How each residence gets 18 billion billion IPv6 addresses
Carrier Packet Networks   •  Technologies   •  MPLS   •  SLAs   •  CoS   •  Integration & Aggregation

MPLS and Carrier Networks is a comprehensive, up-to-date course on the networks companies like AT&T build and operate, how they are implemented, the services they offer, and how customers connect to the network.
The IP packets and routing of the previous course is one part of the story. Performance guarantees, and methods for quality of service, traffic management, aggregation and integration is another big part of the story, particularly once we leave the lab and venture into the real world and the business of telecommunications services.
We'll begin by establishing a basic model for a customer obtaining service from a provider, defining Customer Edge, Provider Edge, access and core, and a Service Level Agreement: traffic profile vs. transmission characteristics.
Next, we'll understand virtual circuits, a powerful tool used for traffic management and how they are implemented with MPLS, explaining the equipment, jargon and principles of operation.
Without bogging down on details, we’ll cut through buzzwords and marketing to demystify:

  • Carrier packet networks and services
  • Customer Edge (CE) and Provider Edge (PE)
  • Service Level Agreements
  • Traffic profiles
  • Virtual circuits
  • QoS, Class of Service and Differentiated Services
  • Integration, convergence and aggregation
  • MPLS and other network technologies
  • How this relates to TCP/IP
  • How MPLS is used for business customer VPNs
  • How MPLS is used for integrated access:
  • How all services are carried together on one circuit
  • How MPLS is used to prioritize and manage IP packets
  • MPLS services" vs. the Internet
This course can be taken by those who need just an introduction to carrier networks and MPLS, as well as by those who need to establish a solid base on which to build more detailed knowledge.
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About TCO Certification

Teracom is an Accredited Training Partner of the Telecommunications Certification Organization, authorized to administer exams for TCO certifications on the myTeracom Learning Management System and award TCO Certifications.

TCO Certification is proof of your knowledge of telecom, datacom and networking fundamentals, jargon, buzzwords, technologies and solutions.

It's backed up with a Certificate suitable for framing - plus a personalized Letter of Reference / Letter of Introduction detailing the knowledge your TCO Certification represents and inviting the recipient to contact Teracom for verification.

You may list Teracom Training Institute as a reference on your résumé if desired.

Getting your Certificate

Each course has a course exam, consisting of ten multiple-choice questions chosen at random from a pool and shuffled in order. Passing the course exams proves your knowledge of these topics and results in your certification as a Certified Telecommunications Network Specialist.

Your Certificate and Letter of Reference / Letter of Introduction will be immediately available for download from your Dashboard in the myTeracom Learning Management System. You may also order a signed and sealed Certificate by airmail.
Choosing the "Unlimited Plan" at registration allows you to repeat courses and/or exams at no additional charge – which means guaranteed to pass if you're willing to learn.

Alternatively, if you like this discounted package of courses, but don't need the certification – or don't feel like writing exams – no problem! Take the Telecom, Datacom and Networking for Non-Engineers course package, which includes the same courses as the CTNS certification package, without the certification exams.

Benefits of Certification for Individuals

One benefit of TCO certification is differentiating yourself from the rest of the crowd when applying for a job or angling for a promotion.

The knowledge you gain taking Teracom's Online Courses, confirmed with TCO Certification, is foundational knowledge in telecommunications, IP, networking and wireless: fundamental concepts, mainstream technologies, jargon, buzzwords, and the underlying ideas - and how it all fits together.

This type of knowledge and preparation makes you an ideal candidate to hire or promote to a task, as you will be able to build on your knowledge base to quickly get up to speed and work on a particular project - then have the versatility to work on subsequent projects.

TCO Certification will help demonstrate you have this skill... a desirable thought to have in your potential manager's mind.

Benefits of Certification for Employers

Take advantage of these courses for individual learning, a team, or for an entire organization.

The scalable myTeracom Learning Management System can register and manage all of your people through their courses, lessons and exams, and generate management reports showing progress and scores with the click of a button.

For larger organizations, the courses and exams can also be licensed and deployed on an organization's internal LMS.

Teracom certification packages are an extremely cost-effective way of implementing consistent, comprehensive telecommunications and networking technology fundamentals training, ensuring that both existing resources and new hires are up to the same speed, with a common vocabulary, framework and knowledge base.

The course exams provide concrete measurements of competency in key knowledge areas. Management can view the progress and results of all team members and export the results to Excel with the click of a button.

These reports identify skills deficiencies and strengths, and provide tangible proof of return on investment and team readiness for reports to upper management.

Teracom Advantages

  • Training based on Teracom's proven instructor-led training courses developed and refined over more than twenty years providing training for organizations including AT&T, Verizon, Bell Canada, Intel, Microsoft, Cisco, Qualcomm, the CIA, NSA, IRS, FAA, US Army, Navy, Marines and Air Force and hundreds of others, Teracom online courses are top-notch, top-quality and right up to date with the topics and knowledge you need.
  • Proven courses used by the biggest telecom carriers to train their employees
    These courses are the same courses used by the biggest telecom carriers in the business to train their employees - constantly updated to deliver the core technical knowledge required in the telecom business today. This is the best quality training of its kind available.
  • GSA Schedule
    Teracom online courses and certification packages are on our US Government supply contract... which took two years and a 200-page application... so you know you are getting quality.
  • 30-Day 100% Money-Back Guarantee
    You are protected by Teracom's 30-day, no-questions-asked, 100% money-back guarantee.  terms and conditions
  • Career-enhancing knowledge
    This training is an ideal way to implement a career-enhancing upgrade to your knowledge, or to prepare for a job in the telecommunications business.
  • Guaranteed to Pass with the Unlimited Plan
    Choose the Unlimited Plan for unlimited repeats of courses and exams - which means you can retake courses to refresh your knowledge in the future, and guaranteed to pass the exam if you're willing to learn.  unlimited plan info
  • Certificate and Letter of Reference
    In addition to your TCO Certificate, you will also receive – a Teracom exclusive – a personalized Letter of Reference / Letter of Introduction explaining the courses you took and the knowledge you have, and inviting anyone you give it to to contact Teracom a reference... an excellent addition to your CV.
  • Self-paced training
    The courses and their lessons can be done at your own pace. There are no time limits for completing a lesson and moving to the next one. The courses may be done in any order.
  • Team training
    These courses are a highly cost-effective and consistent way for managers to get team members up to a common speed with measurable results. The myTeracom Learning Management System provides management reports showing your team's progress with a few clicks of the mouse.  more info

What is the value of the CTNS certification?  Click here to find out