Draytek 2130 manual 3.3 NAT, and MAC address listed in ARP table can be selected and added

Enable	Click this radio button to invoke this function. However,
	IP/MAC which is not listed in IP Bind List also can connect to
	Internet.
Disable	Click this radio button to disable this function. All the settings
	on this page will be invalid.
Strict Bind	Click this radio button to block the connection of the IP/MAC
	which is not listed in IP Bind List.
ARP Table	This table is the LAN ARP table of this router. The information
	for IP and MAC will be displayed in this field. Each pair of IP
	and MAC address listed in ARP table can be selected and added
	to IP Bind List by clicking Add below.
Add and Edit	IP Address – Type the IP address that will be used for the
	specified MAC address.
	Mac Address – Type the MAC address that is used to bind
	with the assigned IP address.
Refresh	It is used to refresh the ARP table. When there is one new PC
	added to the LAN, you can click this link to obtain the newly
	ARP table information.
IP Bind List	It displays a list for the IP bind to MAC information.
Add	It allows you to add the one you choose from the ARP table or
	the IP/MAC address typed in Add and Edit to the table of IP
	Bind List.
Edit	It allows you to edit and modify the selected IP address and
	MAC address that you create before.
Remove	You can remove any item listed in IP Bind List. Simply click
	and select the one, and click Remove. The selected item will be
	removed from the IP Bind List.

Note: Before you select Strict Bind, you have to bind one set of IP/MAC address for one PC. If not, no one of the PCs can access into Internet. And the web configurator of the router might not be accessed.

3.3 NAT

Usually, the router serves as an NAT (Network Address Translation) router. NAT is a mechanism that one or more private IP addresses can be mapped into a single public one. Public IP address is usually assigned by your ISP, for which you may get charged. Private IP addresses are recognized only among internal hosts.

When the outgoing packets destined to some public server on the Internet reach the NAT router, the router will change its source address into the public IP address of the router, select the available public port, and then forward it. At the same time, the router shall list an entry in a table to memorize this address/port-mapping relationship. When the public server response, the incoming traffic, of course, is destined to the router’s public IP address and the router will do the inversion based on its table. Therefore, the internal host can communicate with external host smoothly.

The benefit of the NAT includes:

zSave cost on applying public IP address and apply efficient usage of IP address. NAT allows the internal IP addresses of local hosts to be translated into one public IP address, thus you can have only one IP address on behalf of the entire internal hosts.

50 Vigor2130 Series User’s Guide