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Table Of Contents  CertiGuide to Network+
 9  Chapter 0101:  TCP/IP

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XIV  IP Proxy
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XVI  Classless Inter-Domain Routing
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XV  Network Address Translation

While examining IP addresses and the class structure, we discussed how one private network address was reserved for each class. (As you recall, a private network is one to which only the internal organization has access.) This allows any group of devices to use TCP/IP without connecting to the outside world, and without consuming a public network address. The use of private IP addresses in business has kept the need for more unique numbers from burning out of control.

At the same, the efficiently of a business would be severely hampered if it couldn’t communicate with the world outside of itself. So, a technology known as Network Address Translation (NAT) was developed to allow devices on private networks to communicate with devices in the outside world.

How NAT works

A NAT is set up to provide IP Proxy service. That is, NAT requires a device, such as a NIC that is part of the private network to the interface with the outside world. Another device for example a modem, router, or NIC gets a public IP address. The two pieces are brought together only for the data that needs to bridge to or from the outside world.

Table 9: NAT IP Address Mapping

External IP Address

Internal IP Address

199.97.31.10

192.68.0.5

199.97.31.10

192.68.0.6

199.97.31.10

192.68.0.7


How the bridge is formed depends on the network’s design needs. In a small office environment with simple needs, for example 3 people needing web access, the outside requests are internally kept separate by assigning unique port numbers to each internal IP request. This, the simplest case of NAT, is when only the NAT function is provided, rather than all IP Proxy services. This can be accomplished in Windows via the Internet Connection Sharing (ICS) facility, or by some consumer routers for high-speed Internet connections.

Internet Connection Sharing

Microsoft
provides a version of NAT in its most recent versions of Windows. Internet Connection Sharing, or ICS, enables one computer’s Internet connection (usually via modem, Cable or DSL) to be shared with the other PC’s on the LAN.


Larger environments or any situation with more sophisticated needs utilizes solutions such as a proxy server, which combine NAT with other functionality to improve security and network efficiency. Streaming audio is an example where a straight NAT table will only support one user at a time. As noted before, a proxy server can provide streaming audio to multiple users, have a firewall built-in, allow/deny access based on a great variety of rules, and create reports. While none of this is found in a NAT, the NAT is simple to maintain.

The idea of NAT and its cousin solution, IP proxy server, kept the world from exhausting unique IP numbers years ago. This only reduced the consumption of unique numbers, leaving other issues to be addresses. One huge issue that could have made Dr. Metcalfe’s prediction true had do be corrected, and quickly. One solution has the quirky acronym of CIDR, and reviewed next.


Previous Topic/Section
XIV  IP Proxy
Previous Page
Pages in Current Topic/Section
1
Next Page
XVI  Classless Inter-Domain Routing
Next Topic/Section

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