To reduce the impact of network address depletion due to the rapid growth of the Internet, many large IP networks needed the ability to retain their current addressing scheme yet be able access the Internet. This can be accomplished with Network Address Translation (NAT) defined in RFC 1631.
The implementation of NAT by the Cisco IOS supports most of the applications we have discussed so far, including Domain Name System and File Transfer Protocol. The common applications supported on Cisco routers running NAT include the following:
► HTTP, DNS zone transfers, TFTP, BOOTP, telnet, SNMP, finger, NTP, NFS, rlogin, rsh, rcp
► ICMP, FTP (including PORT and PASV commands), NetBIOS over TCP/IP.
The following URL provides a full list of supported applications: www.cisco.com/cpropart/sync-src/ccstcp/cc/cisco/mkt/ios/nat/prodlit/ 792_pp.htm#xtocid11070
NAT is a standard defined in RFC 1631. Cisco devices started supporting NAT in IOS versions 11.2 and higher. NAT grants the ability to retain a network's original IP addressing scheme while translating that scheme to valid Internet IP addresses. Thus the layer 3 address is changed when the packet is sent out to the Internet and vice versa.
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