Comtrend Corporation NexusLink 3120 User Manual page 198

ENABLE FULLCONE NAT
This option becomes available when NAT is enabled. Known as one-to-one NAT, all
requests from the same internal IP address and port are mapped to the same
external IP address and port. An external host can send a packet to the internal host,
by sending a packet to the mapped external address.
DIAL ON DEMAND
The NexusLink 3120 can be configured to disconnect if there is no activity for a
period of time by selecting the Dial on demand checkbox . You must also enter
an inactivity timeout period in the range of 1 to 4320 minutes.
PPP IP EXTENSION
The PPP IP Extension is a special feature deployed by some service providers.
Unless your service provider specifically requires this setup, do not select it.
PPP IP Extension does the following:
 Allows only one PC on the LAN.
 Disables NAT and Firewall.
 The device becomes the default gateway and DNS server to the PC
through DHCP using the LAN interface IP address.
 The device extends the IP subnet at the remote service provider to the
LAN PC. i.e. the PC becomes a host belonging to the same IP subnet.
 The device bridges the IP packets between WAN and LAN ports, unless
the packet is addressed to the device's LAN IP address.
 The public IP address assigned by the remote side using the PPP/IPCP
protocol is actually not used on the WAN PPP interface. Instead, it is
forwarded to the PC LAN interface through DHCP. Only one PC on the
LAN can be connected to the remote, since the DHCP server within the
device has only a single IP address to assign to a LAN device.
ENABLE NAT
If the LAN is configured with a private IP address, the user should select this
checkbox . The NAT submenu will appear in the Advanced Setup menu after reboot.
On the other hand, if a private IP address is not used on the LAN side (i.e. the LAN
side is using a public IP), this checkbox  should not be selected to free up system
resources for better performance.
ENABLE FIREWALL
If this checkbox  is selected, the Security submenu will be displayed on the
Advanced Setup menu after reboot. If firewall is not necessary, this checkbox 
should not be selected to free up system resources for better performance.
USE STATIC IPv4 ADDRESS
Unless your service provider specially requires it, do not select this checkbox . If
selected, enter the static IP address in the IP Address field. Also, don't forget to
adjust the IP configuration to Static IP Mode as described in
Fixed MTU
Fixed Maximum Transmission Unit. The size (in bytes) of largest protocol data unit
which the layer can pass onwards. This value is 1500 for PPPoA.
Section
197
3.2.