3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 11 NAT Configuration3Com Corporation11-6in the DNS response following the conventional NAT procedures and sends theDNS response to PC 2.2) PC 2 initiates an access to 3.0.0.1, the temporary address for www.web.com.When Router A receives this request, it first translates the source addressfollowing the conventional NAT procedures, and then translates the destinationaddress, which is temporary, to 10.0.0.1, the corresponding overlapping address.3) Router A sends the request out its outgoing interface, and the request is forwardedacross the WAN hop by hop to PC 3.4) When receiving the response from PC 3, Router A checks it and finds out that thesource address 10.0.0.1 is an overlapping address. The router then translates thisaddress to its corresponding temporary address, 3.0.0.1. After translating thedestination address following the conventional NAT procedures, Router A sendsthe response back to PC 2.11.2.5 Internal ServerNAT can “shield” internal hosts via hiding the architecture of the intranet. However,there are times that you want to permit some hosts on external networks to accesssome hosts on the intranet, such as a WWW server or a FTP server. You can flexiblyadd servers on the intranet via NAT, for example, you can use 202.169.10.10 as theexternal address of the WWW server and 202.110.10.11 as the external address of theFTP server. Even 202.110.10.12:8080 can be used as the external address of theWWW server. Moreover, NAT can provide multiple identical servers such as WWWservers for external clients.The NAT function on 3Com Series Routers provides some servers on the intranet forsome hosts on external networks. When a client on an external network accesses aserver on the intranet, the NAT device translates the destination address in the requestpacket into a private address on the internal server and translates the source address(a private address) in the response packet into a public address.11.2.6 Easy IPEasy IP is to use the public IP address of an interface as the source address after theaddress translation. It also controls the address translation based on ACL.11.2.7 NAT Support for ALGNetwork translation may result in malfunction of many application protocols. Theseprotocols are NAT-sensitive. Some of their packets require special treatment on the IPaddress and port number in the valid payload, to ensure normal subsequent protocolinteraction.Application level gateway (ALG) of NAT is a common approach to NAT traversal. Itsubstitutes the IP address and port number in payload according to address translation
