Configure Ethernet interfaces
Digi TransPort User Guide
157
Take this interface out of service after n seconds when the link is lost
(such as cable removed or broken)
The length of time, in seconds, the router will wait after detecting that an Ethernet cable has
been removed before routes that were using that interface are marked as out of service. If the
parameter is set to 0, the feature is disabled such as routes using the interface will not be
marked as out of service if the cable is removed.
Enable NAT on this interface
Selects whether IP Network Address Translation (NAT) or Network Address and Port
Translation (NAPT) are used at the Ethernet interface. When the parameter is set to disabled,
no NAT will take place. When this parameter is enabled, extra options described below will be
displayed.
NAT and NAPT can have many uses but they are generally used to allow a number of private
IP hosts (PCs for example) to connect to the Internet through a single shared public IP address.
This has two main advantages, it saves on IP address space (the ISP only need assign you one
IP address), and it isolates the private IP hosts from the Internet (effectively providing a simple
firewall because unsolicited traffic from the Internet cannot be routed directly to the private IP
hosts.
To use NAT or NAPT correctly in the example of connecting private hosts to the Internet, NAT
or NAPT should be enabled on the router’s WAN side interface and should be disabled on the
router’s LAN side interface.
IP address
When a private IP host sends a UDP or TCP packet to an Internet IP address, the router will
change the source address of the packet from the private host IP to the router’s public IP
address before forwarding the packet onto the Internet host. Additionally it will create an
entry in a
NAT table
containing the private IP source address, the private IP port number, the
public IP destination address and the destination port number. Conversely, when the router
receives a reply packet back from the public host, it checks the source IP, source port number
and destination port number in the NAT table to determine which private host to forward the
packet to. Before it forwards the packet back to the private host, it changes the destination IP
address of the packet from its public IP address to the IP address of the private host.
IP address and Port
This mode behaves like NAT, but in addition to changing the source IP of the packet from the
private host, it can also change the source port number. This is required if more than one
private host attempts to connect using the same local port number to the same Internet host
on the same remote port number. If such a scenario were to occur with NAT the router would
be unable to determine which private host to route the returning packets to and the
connection would fail.
