Network Address Translation (NAT) C-3provides a firewall and hides the private local network from the outsideworld.Figure C–1 NAT ExampleDynamic NAT When a local PC sends a packet destined for the WAN, the OfficeConnectRemote 840 puts the private source IP address and an IP address from thepool into an address translation table. A change is made in the datapacket: the private source IP address is replaced by the IP address fromthe pool and sent to the WAN. When the reply returns, it contains the IPaddress from the pool. This address is used to search the addresstranslation table for the original private IP address. The private IP addressis put into a reply packet and sent to the Ethernet.When all the pool addresses are in use, any new requests are rejected andthe workstation on the LAN has to wait until one of the pool addresses isfree for use. To ensure that addresses are not held indefinitely, a timer isassociated with each table entry. An entry is freed after 5 minutes ofinactivity or if the session between the workstation and the remote site isended.This method requires initiating incoming packets from the LAN. Packetsfrom the WAN are rejected unless they have an IP address number that isin the pool and is currently in the address translation table.840ug.book Page 3 Friday, July 7, 2000 2:23 PM
