This lesson explains the standard practice of assigning private IP addresses to machines inside the building, and getting a single public IP address from the ISP providing the Internet access. Everyone in the building shares the single public IP address via Network Address Translation. This less explains how NAT works.
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Sixteen online courses covering telecom, datacom and networking for non‑engineers from A-Z, plus the prestigious TCO Certified Telecommunications Analyst certification.
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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.
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.
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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.
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:
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:
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.
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:
On completion of this course, you will be able to explain:
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:
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.
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.
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.
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.