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Draytek Vigor 2110F series Manual

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Vigor2110F Series User’s Guide
41
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. DSL/Cable) connection.
There are two common scenarios of LAN settings that stated in Chapter 5. For the
configuration examples, please refer to that chapter to get more information for your necessity.
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.
3
3
.
.
3
3
.
.
1
1
P
P
o
o
r
r
t
t
R
R
e
e
d
d
i
i
r
r
e
e
c
c
t
t
i
i
o
o
n
n
Port Redirection is usually set up for server related service inside the local network (LAN),
such as web servers, FTP servers, E-mail servers etc. Most of the case, you need a public IP
address for each server and this public IP address/domain name are recognized by all users.
Since the server is actually located inside the LAN, the network well protected by NAT of the
router, and identified by its private IP address/port, the goal of Port Redirection function is to
forward all access request with public IP address from external users to the mapping private IP
address/port of the server.
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