Network Address Translation (NAT)

NAT was developed to address a couple of concerns. First, the number of public IP addresses available on the Internet was becoming depleted | Network Address Translation NAT NAT was developed to address a couple of concerns. First the number of public IP addresses available on the Internet was becoming depleted. Second because of the interconnectivity of networks it was possible for an administrator to assign a set of IP addresses to a network that someone else might be using. This is a common situation when two companies and their respective networks are combined. NAT addresses these two concerns by providing a mechanism by which any number of IP addresses can be translated to a different range of IP addresses or in some cases a single or smaller range of IP addresses. To address the limitation of available IP addresses NAT can be used to translate hundreds or even thousands of IP addresses to just a couple of IP addresses or even a single IP address thereby allowing a company to provide Internet access to their hosts without needing to allocate thousands of IP addresses on the Internet to do so. To address the issue of invalid networks or in many cases duplicate networks NAT can be used to allow each network to appear as a completely different network. Figure 3-13 illustrates the process of NAT and Internet connectivity. Figure 3-13. Example of NAT and Internet Access In this example when Host A attempts to access the Internet the firewall translates the request from having a source address of to having a source address of and transmits the data across the Internet. The firewall then stores this translation in its translation table so that it knows how to deal with the return traffic. When host B receives the data it thinks it is communicating with and addresses the return traffic accordingly. When the firewall receives the return traffic it refers back to its translation table and determines that the traffic should be delivered to . The firewall repackages the packet this time changing the destination IP address to be and transmits it .