Digi TransPort WR11 User Manual page 185

Configuring network interfaces
TCP transmit buffer size
When set to a non-zero value, this parameter sets the TCP buffer size of transmitted packets
in bytes. This is useful for slow / lossy connections such as satellite. Setting this buffer to a low
value will prevent the amount of unacknowledged data from getting too high. If retransmits
are required, a smaller TX buffer helps prevent the flooding of the connection by retransmits.
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 and routes using the interface will not be marked
as out of service if the cable is removed.
Enable NAT on this interface
Selects whether the Ethernet interface uses IP, Network Address Translation (NAT), or
Network Address and Port Translation (NAPT). When the parameter is disabled, no NAT occurs.
When enabled, extra options are displayed, described below.
NAT and NAPT have many uses, but the router generally uses them 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
Digi TransPort® Routers User Guide
Configure Ethernet interfaces
185