Comtrend Corporation NexusLink 3120 User Manual page 190

STEP 3: This screen provides access to NAT, Firewall and IGMP Multicast settings.
Enable each by selecting the appropriate checkbox . Click Next to
continue or click Back to return to the previous step.
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, so as to
free up system resources for improved performance.
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.
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 so as to free up system resources for better performance.
ENABLE IGMP MULTICAST PROXY
Tick the checkbox  to enable Internet Group Membership Protocol (IGMP)
multicast. This protocol is used by IPv4 hosts to report their multicast group
memberships to any neighboring multicast routers.
ENABLE IGMP MULTICAST SOURCE
Enable the WAN interface to be used as IGMP multicast source.
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