Backbones

The National Science Foundation (NSF) created the first high-speed backbone in 1987. Called NSFNET, it was a T1 line that connected 170 smaller networks together and operated at 1.544 Mbps (million bits per second). IBM, MCI and Merit worked with NSF to create the backbone and developed a T3 (45 Mbps) backbone the following year.
Backbones are typically fiber optic trunk lines. The trunk line has multiple fiber optic cables combined together to increase the capacity. Fiber optic cables are designated OC for optical carrier, such as OC-3, OC-12 or OC-48. An OC-3 line is capable of transmitting 155 Mbps while an OC-48 can transmit 2,488 Mbps (2.488 Gbps). Compare that to a typical 56K modem transmitting 56,000 bps and you see just how fast a modern backbone is.
Today there are many companies that operate their own high-capacity backbones, and all of them interconnect at various NAPs around the world. In this way, everyone on the Internet, no matter where they are and what company they use, is able to talk to everyone else on the planet. The entire Internet is a gigantic, sprawling agreement between companies to intercommunicate freely.

Bridging The Divide

All of these networks rely on NAPs, backbones and routers to talk to each other. What is incredible about this process is that a message can leave one computer and travel halfway across the world through several different networks and arrive at another computer in a fraction of a second!
The routers determine where to send information from one computer to another. Routers are specialized computers that send your messages and those of every other Internet user speeding to their destinations along thousands of pathways. A router has two separate, but related, jobs:
It ensures that information doesn't go where it's not needed. This is crucial for keeping large volumes of data from clogging the connections of "innocent bystanders."
It makes sure that information does make it to the intended destination.
In performing these two jobs, a router is extremely useful in dealing with two separate computer networks. It joins the two networks, passing information from one to the other. It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other. Regardless of how many networks are attached, the basic operation and function of the router remains the same. Since the Internet is one huge network made up of tens of thousands of smaller networks, its use of routers is an absolute necessity. For more information, read How Routers Work.
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A Network Example

Here's an example. Imagine that Company A is a large ISP. In each major city, Company A has a POP. The POP in each city is a rack full of modems that the ISP's customers dial into. Company A leases fiber optic lines from the phone company to connect the POPs together (see, for example, this UUNET Data Center Connectivity Map).
Imagine that Company B is a corporate ISP. Company B builds large buildings in major cities and corporations locate their Internet server machines in these buildings. Company B is such a large company that it runs its own fiber optic lines between its buildings so that they are all interconnected.
In this arrangement, all of Company A's customers can talk to each other, and all of Company B's customers can talk to each other, but there is no way for Company A's customers and Company B's customers to intercommunicate. Therefore, Company A and Company B both agree to connect to NAPs in various cities, and traffic between the two companies flows between the networks at the NAPs.
In the real Internet, dozens of large Internet providers interconnect at NAPs in various cities, and trillions of bytes of data flow between the individual networks at these points. The Internet is a collection of huge corporate networks that agree to all intercommunicate with each other at the NAPs. In this way, every computer on the Internet connects to every other.
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A Hierarchy of Networks

Every computer that is connected to the Internet is part of a network, even the one in your home. For example, you may use a modem and dial a local number to connect to an Internet Service Provider (ISP). At work, you may be part of a local area network (LAN), but you most likely still connect to the Internet using an ISP that your company has contracted with. When you connect to your ISP, you become part of their network. The ISP may then connect to a larger network and become part of their network. The Internet is simply a network of networks.
Most large communications companies have their own dedicated backbones connecting various regions. In each region, the company has a Point of Presence (POP). The POP is a place for local users to access the company's network, often through a local phone number or dedicated line. The amazing thing here is that there is no overall controlling network. Instead, there are several high-level networks connecting to each other through Network Access Points or NAPs.
When you connect to the Internet, your computer becomes part of a network.

Introduction to How Internet Infrastructure Works

One of the greatest things about the Internet is that nobody really owns it. It is a global collection of networks, both big and small. These networks connect together in many different ways to form the single entity that we know as the Internet. In fact, the very name comes from this idea of interconnected networks.
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Since its beginning in 1969, the Internet has grown from four host computer systems to tens of millions. However, just because nobody owns the Internet, it doesn't mean it is not monitored and maintained in different ways. The Internet Society, a non-profit group established in 1992, oversees the formation of the policies and protocols that define how we use and interact with the Internet.
In this article, you will learn about the basic underlying structure of the Internet. You will learn about domain name servers, network access points and backbones. But first you will learn about how your computer connects to others.

Extras: Dynamic Pages

But what about the Web pages that are dynamic? For example:
Any guest book allows you to enter a message in an HTML form, and the next time the guest book is viewed, the page will contain the new entry.
The whois form at Network Solutions allows you to enter a domain name on a form, and the page returned is different depending on the domain name entered.
Any search engine lets you enter keywords on an HTML form, and then it dynamically creates a page based on the keywords you enter. In all of these cases, the Web server is not simply "looking up a file." It is actually processing information and generating a page based on the specifics of the query. In almost all cases, the Web server is using something called CGI scripts to accomplish this feat. CGI scripts are a topic unto themselves, and are described in the HowStuffWorks article How CGI Scripting Work.
For more information on Web servers and related topics, check out the links on the next page.

Extras: Security

You can see from this description that a Web server can be a pretty simple piece of software. It takes the file name sent in with the GET command, retrieves that file and sends it down the wire to the browser. Even if you take into account all of the code to handle the ports and port connections, you could easily create a C program that implements a simple Web server in less than 500 lines of code. Obviously, a full-blown enterprise-level Web server is more involved, but the basics are very simple.
Most servers add some level of security to the serving process. For example, if you have ever gone to a Web page and had the browser pop up a dialog box asking for your name and password, you have encountered a password-protected page. The server lets the owner of the page maintain a list of names and passwords for those people who are allowed to access the page; the server lets only those people who know the proper password see the page. More advanced servers add further security to allow an encrypted connection between server and browser, so that sensitive information like credit card numbers can be sent on the Internet.
That's really all there is to a Web server that delivers standard, static pages. Static pages are those that do not change unless the creator edits the page

Putting It All Together

Now you know a tremendous amount about the Internet. You know that when you type a URL into a browser, the following steps occur:
The browser breaks the URL into three parts:
The protocol ("http")
The server name ("www.howstuffworks.com")
The file name ("web-server.htm")
The browser communicates with a name server to translate the server name, "www.howstuffworks.com," into an IP address, which it uses to connect to that server machine.
The browser then forms a connection to the Web server at that IP address on port 80.
Following the HTTP protocol, the browser sends a GET request to the server, asking for the file "http://www.howstuffworks.com/web-server.htm." (Note that cookies may be sent from browser to server with the GET request -- see How Internet Cookies Work for details.)
The server sends the HTML text for the Web page to the browser. (Cookies may also be sent from server to browser in the header for the page.)
The browser reads the HTML tags and formats the page onto your screen.

Ports

Any server machine makes its services available to the Internet using numbered ports, one for each service that is available on the server. For example, if a server machine is running a Web server and an FTP server, the Web server would typically be available on port 80, and the FTP server would be available on port 21. Clients connect to a service at a specific IP address and on a specific port.
Each of the most well-known services is available at a well-known port number. Here are some common port numbers:
echo 7
daytime 13
qotd 17 (Quote of the Day)
ftp 21
telnet 23
smtp 25 (Simple Mail Transfer, meaning e-mail)
time 37
nameserver 53
nicname 43 (Who Is)
gopher 70
finger 79
WWW 80 If the server machine accepts connections on a port from the outside world, and if a firewall is not protecting the port, you can connect to the port from anywhere on the Internet and use the service. Note that there is nothing that forces, for example, a Web server to be on port 80. If you were to set up your own machine and load Web server software on it, you could put the Web server on port 918, or any other unused port, if you wanted to. Then, if your machine were known as xxx.yyy.com, someone on the Internet could connect to your server with the URL http://xxx.yyy.com:918. The ":918" explicitly specifies the port number, and would have to be included for someone to reach your server. When no port is specified, the browser simply assumes that the server is using the well-known port 80.

Name Servers

set of servers called domain name servers (DNS) maps the human-readable names to the IP addresses. These servers are simple databases that map names to IP addresses, and they are distributed all over the Internet. Most individual companies, ISPs and universities maintain small name servers to map host names to IP addresses. There are also central name servers that use data supplied by VeriSign to map domain names to IP addresses.
If you type the URL "http://www.howstuffworks.com/web-server.htm" into your browser, your browser extracts the name "www.howstuffworks.com," passes it to a domain name server, and the domain name server returns the correct IP address for www.howstuffworks.com. A number of name servers may be involved to get the right IP address. For example, in the case of www.howstuffworks.com, the name server for the "com" top-level domain will know the IP address for the name server that knows host names, and a separate query to that name server, operated by the HowStuffWorks ISP, may deliver the actual IP address for the HowStuffWorks server machine.
On a UNIX machine, you can access the same service using the nslookup command. Simply type a name like "www.howstuffworks.com" into the command line, and the command will query the name servers and deliver the corresponding IP address to you.
So here it is: The Internet is made up of millions of machines, each with a unique IP address. Many of these machines are server machines, meaning that they provide services to other machines on the Internet. You have heard of many of these servers: e-mail servers, Web servers, FTP servers, Gopher servers and Telnet servers, to name a few. All of these are provided by server machines.

Domain Names

Because most people have trouble remembering the strings of numbers that make up IP addresses, and because IP addresses sometimes need to change, all servers on the Internet also have human-readable names, called domain names. For example, www.howstuffworks.com is a permanent, human-readable name. It is easier for most of us to remember www.howstuffworks.com than it is to remember 209.116.69.66.
The name www.howstuffworks.com actually has three parts:
The host name ("www")
The domain name ("howstuffworks")
The top-level domain name ("com") Domain names within the ".com" domain are managed by the registrar called VeriSign. VeriSign also manages ".net" domain names. Other registrars (like RegistryPro, NeuLevel and Public Interest Registry) manage the other domains (like .pro, .biz and .org). VeriSign creates the top-level domain names and guarantees that all names within a top-level domain are unique. VeriSign also maintains contact information for each site and runs the "whois" database. The host name is created by the company hosting the domain. "www" is a very common host name, but many places now either omit it or replace it with a different host name that indicates a specific area of the site. For example, in encarta.msn.com, the domain name for Microsoft's Encarta encyclopedia, "encarta" is designated as the host name instead of "www."

IP Addresses

To keep all of these machines straight, each machine on the Internet is assigned a unique address called an IP address. IP stands for Internet protocol, and these addresses are 32-bit numbers, normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this:
216.27.61.137
The four numbers in an IP address are called octets because they can have values between 0 and 255, which is 28 possibilities per octet.
Every machine on the Internet has a unique IP address. A server has a static IP address that does not change very often. A home machine that is dialing up through a modem often has an IP address that is assigned by the ISP when the machine dials in. That IP address is unique for that session -- it may be different the next time the machine dials in. This way, an ISP only needs one IP address for each modem it supports, rather than for each customer.
If you are working on a Windows machine, you can view a lot of the Internet information for your machine, including your current IP address and hostname, with the command WINIPCFG.EXE (IPCONFIG.EXE for Windows 2000/XP). On a UNIX machine, type nslookup at the command prompt, along with a machine name, like www.howstuffworks.com -- e.g. "nslookup www.howstuffworks.com" -- to display the IP address of the machine, and you can use the command hostname to learn the name of your machine. (For more information on IP addresses, see IANA.)
As far as the Internet's machines are concerned, an IP address is all you need to talk to a server. For example, in your browser, you can type the URL http://209.116.69.66 and arrive at the machine that contains the Web server for HowStuffWorks. On some servers, the IP address alone is not sufficient, but on most large servers it is -- keep reading for details.

Clients and Servers

In general, all of the machines on the Internet can be categorized as two types: servers and clients. Those machines that provide services (like Web servers or FTP servers) to other machines are servers. And the machines that are used to connect to those services are clients. When you connect to Yahoo! at www.yahoo.com to read a page, Yahoo! is providing a machine (probably a cluster of very large machines), for use on the Internet, to service your request. Yahoo! is providing a server. Your machine, on the other hand, is probably providing no services to anyone else on the Internet. Therefore, it is a user machine, also known as a client. It is possible and common for a machine to be both a server and a client, but for our purposes here you can think of most machines as one or the other.
A server machine may provide one or more services on the Internet. For example, a server machine might have software running on it that allows it to act as a Web server, an e-mail server and an FTP server. Clients that come to a server machine do so with a specific intent, so clients direct their requests to a specific software server running on the overall server machine. For example, if you are running a Web browser on your machine, it will most likely want to talk to the Web server on the server machine. Your Telnet application will want to talk to the Telnet server, your e-mail application will talk to the e-mail server, and so on...

The Internet

So what is "the Internet"? The Internet is a gigantic collection of millions of computers, all linked together on a computer network. The network allows all of the computers to communicate with one another. A home computer may be linked to the Internet using a phone-line modem, DSL or cable modem that talks to an Internet service provider (ISP). A computer in a business or university will usually have a network interface card (NIC) that directly connects it to a local area network (LAN) inside the business. The business can then connect its LAN to an ISP using a high-speed phone line like a T1 line. A T1 line can handle approximately 1.5 million bits per second, while a normal phone line using a modem can typically handle 30,000 to 50,000 bits per second.
ISPs then connect to larger ISPs, and the largest ISPs maintain fiber-optic "backbones" for an entire nation or region. Backbones around the world are connected through fiber-optic lines, undersea cables or satellite links (see An Atlas of Cyberspaces for some interesting backbone maps). In this way, every computer on the Internet is connected to every other computer on the Internet.
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Behind the Scenes

If you want to get into a bit more detail on the process of getting a Web page onto your computer screen, here are the basic steps that occurred behind the scenes:
The browser broke the URL into three parts:
The protocol ("http")
The server name ("www.howstuffworks.com")
The file name ("web-server.htm")
The browser communicated with a name server to translate the server name "www.howstuffworks.com" into an IP Address, which it uses to connect to the server machine.
The browser then formed a connection to the server at that IP address on port 80. (We'll discuss ports later in this article.)
Following the HTTP protocol, the browser sent a GET request to the server, asking for the file "http://www.howstuffworks.com/web-server.htm." (Note that cookies may be sent from browser to server with the GET request -- see How Internet Cookies Work for details.)
The server then sent the HTML text for the Web page to the browser. (Cookies may also be sent from server to browser in the header for the page.)
The browser read the HTML tags and formatted the page onto your screen. If you've never explored this process before, that's a lot of new vocabulary. To understand this whole process in detail, you need to learn about IP addresses, ports, protocols... The following sections will lead you through a complete explanation.

The Basic Process

Let's say that you are sitting at your computer, surfing the Web, and you get a call from a friend who says, "I just read a great article! Type in this URL and check it out. It's at http://www.howstuffworks.com/web-server.htm." So you type that URL into your browser and press return. And magically, no matter where in the world that URL lives, the page pops up on your screen.
At the most basic level possible, the following diagram shows the steps that brought that page to your screen:
Your browser formed a connection to a Web server, requested a page and received it.
On the next page, we'll dig a bit deeper.

Introduction to How Web Servers Work

Have you ever wondered about the mechanisms that delivered this page to you? Chances are you are sitting at a computer right now, viewing this page in a browser. So, when you clicked on the link for this page, or typed in its URL (uniform resource locator), what happened behind the scenes to bring this page onto your screen?
If you've ever been curious about the process, or have ever wanted to know some of the specific mechanisms that allow you to surf the Internet, then read on. In this article, you will learn how Web servers bring pages into your home, school or office. Let's get started!

Multi-homing

As businesses rely more and more on the Internet, having multiple points of connection to the Internet is fast becoming an integral part of their network strategy. Multiple connections, known as multi-homing, reduces the chance of a potentially catastrophic shutdown if one of the connections should fail.
In addition to maintaining a reliable connection, multi-homing allows a company to perform load-balancing by lowering the number of computers connecting to the Internet through any single connection. Distributing the load through multiple connections optimizes the performance and can significantly decrease wait times.
Multi-homed networks are often connected to several different ISPs (Internet Service Providers). Each ISP assigns an IP address (or range of IP addresses) to the company. Routers use BGP (Border Gateway Protocol), a part of the TCP/IP protocol suite, to route between networks using different protocols. In a multi-homed network, the router utilizes IBGP (Internal Border Gateway Protocol) on the stub domain side, and EBGP (External Border Gateway Protocol) to communicate with other routers.
Multi-homing really makes a difference if one of the connections to an ISP fails. As soon as the router assigned to connect to that ISP determines that the connection is down, it will reroute all data through one of the other routers.
NAT can be used to facilitate scalable routing for multi-homed, multi-provider connectivity. For more on multi-homing, see Cisco: Enabling Enterprise Multihoming.
For lots more information on NAT and related topics, check out the links on the next page.

Security and Administration

Implementing dynamic NAT automatically creates a firewall between your internal network and outside networks, or between your internal network and the Internet. NAT only allows connections that originate inside the stub domain. Essentially, this means that a computer on an external network cannot connect to your computer unless your computer has initiated the contact. You can browse the Internet and connect to a site, and even download a file; but somebody else cannot latch onto your IP address and use it to connect to a port on your computer.
In specific circumstances, Static NAT, also called inbound mapping, allows external devices to initiate connections to computers on the stub domain. For instance, if you wish to go from an inside global address to a specific inside local address that is assigned to your Web server, Static NAT would enable the connection.
Static NAT (inbound mapping) allows a computer on the stub domain to maintain a specific address when communicating with devices outside the network.
Some NAT routers provide for extensive filtering and traffic logging. Filtering allows your company to control what type of sites employees visit on the Web, preventing them from viewing questionable material. You can use traffic logging to create a log file of what sites are visited and generate various reports from it.
NAT is sometimes confused with proxy servers, but there are definite differences between them. NAT is transparent to the source and to destination computers. Neither one realizes that it is dealing with a third device. But a proxy server is not transparent. The source computer knows that it is making a request to the proxy server and must be configured to do so. The destination computer thinks that the proxy server IS the source computer, and deals with it directly. Also, proxy servers usually work at layer 4 (transport) of the OSI Reference Model or higher, while NAT is a layer 3 (network) protocol. Working at a higher layer makes proxy servers slower than NAT devices in most cases.
NAT operates at the Network layer (layer 3) of the OSI Reference Model -- this is the layer that routers work at.
A real benefit of NAT is apparent in network administration. For example, you can move your Web server or FTP server to another host computer without having to worry about broken links. Simply change the inbound mapping at the router to reflect the new host. You can also make changes to your internal network easily, because the only external IP address either belongs to the router or comes from a pool of global addresses.
NAT and DHCP (dynamic host configuration protocol ) are a natural fit. You can choose a range of unregistered IP addresses for your stub domain and have the DHCP server dole them out as necessary. It also makes it much easier to scale up your network as your needs grow. You don't have to request more IP addresses from IANA. Instead, you can just increase the range of available IP addresses configured in DHCP to immediately have room for additional computers on your network.

Stub Domains

Look at this table to see how the computers on a stub domain might appear to external networks.
SourceComputer
SourceComputer'sIP Address
SourceComputer'sPort
NAT Router'sIP Address
NAT Router'sAssignedPort Number
A
192.168.32.10
400
215.37.32.203
1
B
192.168.32.13
50
215.37.32.203
2
C
192.168.32.15
3750
215.37.32.203
3
D
192.168.32.18
206
215.37.32.203
4
As you can see, the NAT router stores the IP address and port number of each computer in the address translation table. It then replaces the IP address with it's own registered IP address and the port number corresponding to the location, in the table, of the entry for that packet's source computer. So any external network sees the NAT router's IP address and the port number assigned by the router as the source-computer information on each packet.
You can still have some computers on the stub domain that use dedicated IP addresses. You can create an access list of IP addresses that tells the router which computers on the network require NAT. All other IP addresses will pass through untranslated.
The number of simultaneous translations that a router will support are determined mainly by the amount of DRAM (Dynamic Random Access Memory) it has. But since a typical entry in the address-translation table only takes about 160 bytes, a router with 4 MB of DRAM could theoretically process 26,214 simultaneous translations, which is more than enough for most applications.
IANA has set aside specific ranges of IP addresses for use as non-routable, internal network addresses. These addresses are considered unregistered (for more information check out RFC 1918: Address Allocation for Private Internets, which defines these address ranges). No company or agency can claim ownership of unregistered addresses or use them on public computers. Routers are designed to discard (instead of forward) unregistered addresses. What this means is that a packet from a computer with an unregistered address could reach a registered destination computer, but the reply would be discarded by the first router it came to.
There is a range for each of the three classes of IP addresses used for networking:
Range 1: Class A - 10.0.0.0 through 10.255.255.255
Range 2: Class B - 172.16.0.0 through 172.31.255.255
Range 3: Class C - 192.168.0.0 through 192.168.255.255 Although each range is in a different class, your are not required to use any particular range for your internal network. It is a good practice, though, because it greatly diminishes the chance of an IP address conflict.

Dynamic NAT and Overloading

An internal network (stub domain) has been set up with IP addresses that were not specifically allocated to that company by IANA (Internet Assigned Numbers Authority), the global authority that hands out IP addresses. These addresses should be considered non-routable since they are not unique.
The company sets up a NAT-enabled router. The router has a range of unique IP addresses given to the company by IANA.
A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server.
The router receives the packet from the computer on the stub domain.
The router saves the computer's non-routable IP address to an address translation table. The router replaces the sending computer's non-routable IP address with the first available IP address out of the range of unique IP addresses. The translation table now has a mapping of the computer's non-routable IP address matched with the one of the unique IP addresses.
When a packet comes back from the destination computer, the router checks the destination address on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address to the one saved in the address translation table and sends it to that computer. If it doesn't find a match in the table, it drops the packet.
The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.
Here's how overloading works:
An internal network (stub domain) has been set up with non-routable IP addresses that were not specifically allocated to that company by IANA.
The company sets up a NAT-enabled router. The router has a unique IP address given to the company by IANA.
A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server.
The router receives the packet from the computer on the stub domain.
The router saves the computer's non-routable IP address and port number to an address translation table. The router replaces the sending computer's non-routable IP address with the router's IP address. The router replaces the sending computer's source port with the port number that matches where the router saved the sending computer's address information in the address translation table. The translation table now has a mapping of the computer's non-routable IP address and port number along with the router's IP address.
When a packet comes back from the destination computer, the router checks the destination port on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address and destination port to the ones saved in the address translation table and sends it to that computer.
The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.
Since the NAT router now has the computer's source address and source port saved to the address translation table, it will continue to use that same port number for the duration of the connection. A timer is reset each time the router accesses an entry in the table. If the entry is not accessed again before the timer expires, the entry is removed from the table

NAT Configuration

NAT can be configured in various ways. In the example below, the NAT router is configured to translate unregistered (inside, local) IP addresses, that reside on the private (inside) network, to registered IP addresses. This happens whenever a device on the inside with an unregistered address needs to communicate with the public (outside) network.
An ISP assigns a range of IP addresses to your company. The assigned block of addresses are registered, unique IP addresses and are called inside global addresses. Unregistered, private IP addresses are split into two groups. One is a small group (outside local addresses) that will be used by the NAT routers. The other, much larger group, known as inside local addresses, will be used on the stub domain. The outside local addresses are used to translate the unique IP addresses, known as outside global addresses, of devices on the public network.
IP addresses have different designations based on whether they are on the private network (stub domain) or on the public network (Internet), and whether the traffic is incoming or outgoing.
Most computers on the stub domain communicate with each other using the inside local addresses.
Some computers on the stub domain communicate a lot outside the network. These computers have inside global addresses, which means that they do not require translation.
When a computer on the stub domain that has an inside local address wants to communicate outside the network, the packet goes to one of the NAT routers.
The NAT router checks the routing table to see if it has an entry for the destination address. If it does, the NAT router then translates the packet and creates an entry for it in the address translation table. If the destination address is not in the routing table, the packet is dropped.
Using an inside global address, the router sends the packet on to it's destination.
A computer on the public network sends a packet to the private network. The source address on the packet is an outside global address. The destination address is an inside global address.
The NAT router looks at the address translation table and determines that the destination address is in there, mapped to a computer on the stub domain.
The NAT router translates the inside global address of the packet to the inside local address, and sends it to the destination computer.
NAT overloading utilizes a feature of the TCP/IP protocol stack, multiplexing, that allows a computer to maintain several concurrent connections with a remote computer (or computers) using different TCP or UDP ports. An IP packet has a header that contains the following information:
Source Address - The IP address of the originating computer, such as 201.3.83.132
Source Port - The TCP or UDP port number assigned by the originating computer for this packet, such as Port 1080
Destination Address - The IP address of the receiving computer, such as 145.51.18.223
Destination Port - The TCP or UDP port number that the originating computer is asking the receiving computer to open, such as Port 3021
The addresses specify the two machines at each end, while the port numbers ensure that the connection between the two computers has a unique identifier. The combination of these four numbers defines a single TCP/IP connection. Each port number uses 16 bits, which means that there are a possible 65,536 (216) values. Realistically, since different manufacturers map the ports in slightly different ways, you can expect to have about 4,000 ports available.

What Does NAT Do?

NAT is like the receptionist in a large office. Let's say you have left instructions with the receptionist not to forward any calls to you unless you request it. Later on, you call a potential client and leave a message for that client to call you back. You tell the receptionist that you are expecting a call from this client and to put her through.
The client calls the main number to your office, which is the only number the client knows. When the client tells the receptionist that she is looking for you, the receptionist checks a lookup table that matches your name with your extension. The receptionist knows that you requested this call, and therefore forwards the caller to your extension.
Developed by Cisco, Network Address Translation is used by a device (firewall, router or computer) that sits between an internal network and the rest of the world. NAT has many forms and can work in several ways:
Static NAT - Mapping an unregistered IP address to a registered IP address on a one-to-one basis. Particularly useful when a device needs to be accessible from outside the network.
In static NAT, the computer with the IP address of 192.168.32.10 will always translate to 213.18.123.110.
Dynamic NAT - Maps an unregistered IP address to a registered IP address from a group of registered IP addresses.
In dynamic NAT, the computer with the IP address 192.168.32.10 will translate to the first available address in the range from 213.18.123.100 to 213.18.123.150.
Overloading - A form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address by using different ports. This is known also as PAT (Port Address Translation), single address NAT or port-level multiplexed NAT.
In overloading, each computer on the private network is translated to the same IP address (213.18.123.100), but with a different port number assignment.
Overlapping - When the IP addresses used on your internal network are registered IP addresses in use on another network, the router must maintain a lookup table of these addresses so that it can intercept them and replace them with registered unique IP addresses. It is important to note that the NAT router must translate the "internal" addresses to registered unique addresses as well as translate the "external" registered addresses to addresses that are unique to the private network. This can be done either through static NAT or by using DNS and implementing dynamic NAT.
The internal IP range (237.16.32.xx) is also a registered range used by another network. Therefore, the router is translating the addresses to avoid a potential conflict with another network. It will also translate the registered global IP addresses back to the unregistered local IP addresses when information is sent to the internal network.
The internal network is usually a LAN (Local Area Network), commonly referred to as the stub domain. A stub domain is a LAN that uses IP addresses internally. Most of the network traffic in a stub domain is local, so it doesn't travel outside the internal network. A stub domain can include both registered and unregistered IP addresses. Of course, any computers that use unregistered IP addresses must use Network Address Translation to communicate with the rest of the world. In the next section we'll look at the different ways NAT can be configured.

How Network Address Translation Works

Introduction to How Network Address Translation Works

If you are reading this article, you are most likely connected to the Internet and viewing it at the HowStuffWorks Web site. There's a very good chance that you are using Network Address Translation (NAT) right now.
The Internet has grown larger than anyone ever imagined it could be. Although the exact size is unknown, the current estimate is that there are about 100 million hosts and more than 350 million users actively on the Internet. That is more than the entire population of the United States! In fact, the rate of growth has been such that the Internet is effectively doubling in size each year.
So what does the size of the Internet have to do with NAT? Everything! For a computer to communicate with other computers and Web servers on the Internet, it must have an IP address. An IP address (IP stands for Internet Protocol) is a unique 32-bit number that identifies the location of your computer on a network. Basically, it works like your street address -- as a way to find out exactly where you are and deliver information to you.
When IP addressing first came out, everyone thought that there were plenty of addresses to cover any need. Theoretically, you could have 4,294,967,296 unique addresses (232). The actual number of available addresses is smaller (somewhere between 3.2 and 3.3 billion) because of the way that the addresses are separated into classes, and because some addresses are set aside for multicasting, testing or other special uses.
With the explosion of the Internet and the increase in home networks and business networks, the number of available IP addresses is simply not enough. The obvious solution is to redesign the address format to allow for more possible addresses. This is being developed (called IPv6), but will take several years to implement because it requires modification of the entire infrastructure of the Internet.
The NAT router translates traffic coming into and leaving the private network.
This is where NAT (RFC 1631) comes to the rescue. Network Address Translation allows a single device, such as a router, to act as an agent between the Internet (or "public network") and a local (or "private") network. This means that only a single, unique IP address is required to represent an entire group of computers.
But the shortage of IP addresses is only one reason to use NAT. In this edition of HowStuffWorks, you will learn more about how NAT can benefit you. But first, let's take a closer look at NAT and exactly what it can do...

How Domain Name Servers Work

1. Introduction to How Domain Name Servers Work
If you spend any time on the internet sending e-mail or browsing the Web, then you use domain name servers without even realizing it. Domain name servers, or DNS, are an incredibly important but completely hidden part of the internet, and they are fascinating. The DNS system forms one of the largest and most active distributed databases on the planet. Without DNS, the internet would shut down very quickly.
In this article, we’ll take a look at the DNS system so you can understand how it works and appreciate its amazing capabilities.
When you use the web or send an e-mail message, you use a domain name domain name to do it. For example, the URL http://www.lannwan.blogspot.com contains the domain name is lannwan.blogspot.com so does the e-mail address “feedback@lannwan.blogspot.com.:
Human-readable names like “lannwan.blogspot.com”are easy for people to remember, but they don’t do machines any good. All of the machines use names called IP addresses to refer to one another. For example, the machine that humans refer to as www.lannwan.blogspot.com has the IP ADRESS 218.185.123.150. Every time you use a domain name, you use the internets into the machine-readable IP address. During a day of browsing and e-mailing, You might access the domain name servers hundreds of times!
Domain name servers translate domain names to IP addresses. That sounds like a simple task, and it would be – except for five things:
There are billions of IP addresses currently in use, and most machines have a human-readable name as well.
There are many billions of DNS requests mad every day. A single person can easily make a hundred or more DNS requests a day, and there are hundreds of millions people and machines using the internet daily.
Domain names and IP addresses change daily.
New domain names get created daily.
Millions of people do the work to change and domain names and IP addresses every day.The DNS system is a database, and no other database on the planet gets this many requests. No other database on the planet has millions of people changing it every day, either. That is what makes the DNS system so unique

What is IP?

An Identifier for a computer or device on a TCCP/IP network. Networks using the TCP/IP protocol route messages based on the IP address of the destination. The format of an IP address is a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255. For example, 1.160.10.240 could be an IP address.
Within an isolated network, you can assign IP addresses at random as long as each one is unique. However, connecting a private network to the Internet requires using registered IP address (called Internet addresses to avoid duplicates.
The four numbers in an IP address are use in different ways to identify a particular network and host on that network. Four regional Internet registries—ARIN RIPENCC LACNIC AND APNIC – assign Internet addresses for the following three classes.
Class A - supports 16 million hosts on each of 126 networks
Class B - supports 65,000 hosts on each of 16,000 networks
Class C – supports 254 hosts on each of 2 million networks
The number of unassigned Internet addresses is running out, so a new classless scheme called CIDR is gradually replacing the system based on classes A, B, and C and is tied to adoption of IPv6.