Netopia 4541 User Reference Manual

Also see for 4541: Specifications

Contents

Netopia 4541 User Reference Manual Manual pdf 87 page image

Multiple Network Address Translation 9-3When addresses are returned to the group of available addresses, they are returned to the head of the group,being the most recently used. If that same host requests a connection an hour later, and the same publicaddress is still available, then it will be mapped to the same private host. If a new host, which has notpreviously requested a connection, initiates a connection it is allocated the last, or oldest, public addressavailable.Dynamic NAT is a way of sharing a range of public, or exterior, NAT addresses among one or more groups ofprivate, or interior, hosts. This is intended to provide superior support for applications that traditionally havedifficulty communicating through NAT. Dynamic NAT is intended to provide functionality beyond many-to-one andone-to-one translation. Netopia’s NAT implementation makes it possible to have a static mapping of one publicaddress to one private address, thus allowing applications such as NetMeeting to work by assuring that anytraffic sent back to the source IP address is forwarded through to the internal machine.Static one-to-one mapping works well if you have enough IP addresses for all the workstations on your LAN. Ifyou do not, Dynamic NAT allows machines to make full use of the publicly routable IP addresses provided by theISP as necessary, on demand. When these public IP addresses are no longer being used by a particularworkstation, they are returned to a pool of available addresses for other workstations to use.A common example is a DSL customer’s application. Most DSL ISPs only provide customers with a few IPaddresses for use on their network. For networks with more than four or five machines it is usually mandatory touse NAT. A customer may have 15 workstations on the LAN, all of which need Internet access. The customer isonly provided five IP addresses by their ISP. The customer has eight hosts, which only need to use email andhave Web access, but another seven hosts, which use NetMeeting to communicate with clients once or twice aday. NetMeeting will not work unless a static one-to-one mapping exists for the machine running NetMeeting touse for communication. The customer does not have enough IP addresses to create a one-to-one mapping foreach of the seven users. This is where dynamic NAT applies.The customer can configure four of these addresses to be used for Dynamic NAT. The fifth address is then usedfor the eight other machines that do not need one-to-one mappings. As each machine configured to useaddresses from the dynamic pool tries to connect to the Internet it is allocated a public IP address to usetemporarily. Once the communication has been terminated, that IP address is freed for one of the other sixhosts to use.172.16.1.25172.16.1.26172.16.1.27172.16.1.28172.16.1.29Network Address TranslationAvailable for Dynamic NAT Used for Normal NAT192.168.1.16192.168.1.15192.168.1.14192.168.1.13192.168.1.12192.168.1.11192.168.1.10192.168.1.9192.168.1.8192.168.1.7192.168.1.6192.168.1.5192.168.1.4192.168.1.3192.168.1.2WAN NetworkLAN Network