How Network Address Translation (NAT) works

Introduction

If you are reading this, you are most likely connected to the Internet and 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 over 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 just like your street address: 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 the need to set aside some of the addresses for multicasting, testing or other specific 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 (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. Basically, Network Address Translation allows a single device, such as a router, to act as 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 to anything outside their network.
The shortage of IP addresses is only one reason to use NAT. Two other good reasons are:

• Security
• Administration

For more detailed explaination on NAT visit

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094831.shtml#behindmask

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Cisco IOS Software Features and Functions

Cisco IOS Software is the industry-leading and is the most widely deployed network system software. This topic describes the features and functions of Cisco IOS Software. The Cisco IOS Software platform is implemented on most Cisco hardware platforms, including switches, routers, and similar Cisco IOS–based network devices. It is the embedded software architecture in all Cisco devices and is also the operating system of Cisco Catalyst switches.

Cisco IOS Software enables the following network services in Cisco products:

■ Features to carry the chosen network protocols and functions.

■ Connectivity enables high-speed traffic between devices.

■ Security controls access and prohibit unauthorized network use.

■ Scalability adds interfaces and capability as needed for network growth.

■ Reliability ensures dependable access to networked resources.

The Cisco IOS Software command-line interface (CLI) is accessed through a console connection, a modem connection, or a Telnet session. Regardless of which connection method is used, access to the Cisco IOS software CLI is generally referred to as an EXEC session.

SUMMARY of Cisco IOS

■ Cisco IOS Software is embedded software architecture in all the Cisco IOS devices and 

is also the operating system of Catalyst switches. Its functions include carrying the 

chosen network protocols, connectivity, security, scalability, and reliability.

■ A switch or IOS device can be configured from a local terminal connected to the 

console (CON) port, from a remote terminal connected through a modem connection 

to the auxiliary (AUX) port, or through a Telnet (VTY) connection.

■ The CLI is used by network administrators to monitor and configure various Cisco IOS 

devices. The CLI also offers a help facility to aid network administrators with the 

verification and configuration of commands.

■ The CLI supports two EXEC modes: user EXEC mode and privileged EXEC mode. 

The privileged EXEC mode provides more functionality than the user EXEC mode, 

and privileged EXEC mode is also sometimes called enable mode.

■ Cisco IOS devices use Cisco IOS Software with extensive command-line input help 

facilities, including context-sensitive help.

■ The Cisco IOS CLI includes an enhanced editing mode that provides a set of editing 

key functions.

■ A Cisco IOS device's CLI provides a history or record of the commands that have been 

entered.

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Layer 1/2/3 Devices and their Function

Layer 1 - Devices and Their Functions

Layer 1 defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Some common examples are Ethernet segments and serial links like Frame Relay and T1.

Repeaters that provide signal amplification are also considered Layer 1 devices.

The physical interface on the NIC can also be considered part of Layer 1.

Layer 2 Devices and Their Functions

Layer 2 defines how data is formatted for transmission and how access to the physical media is controlled. These devices also provide an interface between the Layer 2 device and the physical media. Some common examples are a NIC installed in a host, bridge, or switch.

Layer 3 Devices and Their Functions

The network layer provides connectivity and path selection between two host systems that might be located on geographically separated networks. In the case of a host, this is the path between the data link layer and the upper layers of the NOS. In the case of a router, it is the actual path across the network.

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Domain Name System

DNS is a mechanism for converting symbolic names into IP addresses. The DNS application frees users of IP networks from the burden of having to remember IP addresses. Without this freedom, the Internet would

not be as popular or as usable as it is. The DNS address is a server that provides the DNS services. The address is typically assigned during the DCHP address assignment or can be assigned manually.

To determine the actual address of the device, the command ipconfig can be used from the command line to display all current TCP/IP network configuration values and refresh DHCP and DNS settings. Used without parameters, ipconfig displays the IP address, subnet mask, and default gateway for all adapters. Figure 1-40 shows an example of an IPCONFIG output.

IPCONFIG Output

You can run ipconfig with various flags to determine exactly what output should be displayed. The syntax flags are as follows:

ipconfig [/all] [/renew [Adapter]] [/release [Adapter]] [/flushdns] [/displaydns] [/registerdns] [/showclassid Adapter] [/setclassid Adapter [ClassID]]

The parameters are as follows:

■ /all: Displays the full TCP/IP configuration for all adapters. Without this parameter, ipconfig displays only the IP address, subnet mask, and default gateway values for each adapter. Adapters can represent physical interfaces, such as installed network adapters, or logical interfaces, such as dialup connections.

■ /renew [Adapter]: Renews DHCP configuration for all adapters (if an adapter is not specified) or for a specific adapter if the Adapter parameter is included. This parameter is available only on computers with adapters that are configured to obtain an IP address automatically. To specify an adapter name, type the adapter name that appears when you use ipconfig without parameters.

■ /release [Adapter]: Sends a DHCPRELEASE message to the DHCP server to release the current DHCP configuration and discard the IP address configuration for either all adapters (if an adapter is not specified) or for a specific adapter if the Adapter parameter is included. This parameter disables TCP/IP for adapters configured to obtain an IP address automatically. To specify an adapter name, type the adapter name that appears when you use ipconfig without parameters.

■ /flushdns: Flushes and resets the contents of the DNS client resolver cache. During DNS troubleshooting, you can use this procedure to discard negative cache entries from the cache, as well as any other entries that have been added dynamically.

■ /displaydns: Displays the contents of the DNS client resolver cache, which includes both entries preloaded from the local hosts file and any recently obtained resource records for name queries resolved by the computer. The DNS client service uses this information to resolve frequently queried names quickly, before querying its configured DNS servers.

■ /registerdns: Initiates manual dynamic registration for the DNS names and IP addresses that are configured at a computer. You can use this parameter to troubleshoot a failed DNS name registration or resolve a dynamic update problem between a client and the DNS server without rebooting the client computer. The DNS settings in the advanced properties of the TCP/IP protocol determine which names are registered in DNS.

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Inter-Network Interconnection

Which states the relationship between 2 pieces or more network devices, using a special device, namely:
Card Connector Network (Network Interface Card = NIC)
 A NIC is a printed circuit board that provides network communication capabilities to and from a personal computer on a network. 

Hub (concentrator)

Repeater

Router (Cisco-Route 2501)

 Bridge

 Gateway

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