Vigor2920 Series User’s Guide
52
one DNS Server. If your ISP does not provide it, the router will
automatically apply default secondary DNS Server IP address:
194.98.0.1 to this field.
The default DNS Server IP address can be found via Online
Status:
If both the Primary IP and Secondary IP Address fields are left
empty, the router will assign its own IP address to local users as
a DNS proxy server and maintain a DNS cache.
If the IP address of a domain name is already in the DNS cache,
the router will resolve the domain name immediately. Otherwise,
the router forwards the DNS query packet to the external DNS
server by establishing a WAN (e.g. Cable) connection.
3
3
.
.
3
3
N
N
A
A
T
T
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:
z Save 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.
z Enhance security of the internal network by obscuring the IP address. There are
many attacks aiming victims based on the IP address. Since the attacker cannot be aware
of any private IP addresses, the NAT function can protect the internal network.
On NAT page, you will see the private IP address defined in RFC-1918. Usually we use the
192.168.1.0/24 subnet for the router. As stated before, the NAT facility can map one or
more IP addresses and/or service ports into different specified services. In other words, the
NAT function can be achieved by using port mapping methods.
Below shows the menu items for NAT.
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