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Page 116 / 249 Scroll up to view Page 111 - 115
Chapter 15 DHCP Server
NBG6515 User’s Guide
116
DNS Servers
Assigned by
DHCP Server
The NBG passes a DNS (Domain Name System) server IP address (in the order you specify
here) to the DHCP clients. The NBG only passes this information to the LAN DHCP clients
when you select the
Enable DHCP Server
check box. When you clear the
Enable DHCP
Server
check box, DHCP service is disabled and you must have another DHCP sever on your
LAN, or else the computers must have their DNS server addresses manually configured.
First DNS
Server
Second DNS
Server
Select
From ISP
if your ISP dynamically assigns DNS server information (and the NBG's
WAN IP address). The field to the right displays the (read-only) DNS server IP address that
the ISP assigns.
Select
User-Defined
if you have the IP address of a DNS server. Enter the DNS server's IP
address in the field to the right. If you chose
User-Defined
, but leave the IP address set to
0.0.0.0,
User-Defined
changes to
None
after you click
Apply
. If you set a second choice to
User-Defined
, and enter the same IP address, the second
User-Defined
changes to
None
after you click
Apply
.
Select
DNS Relay
to have the NBG act as a DNS proxy. The NBG's LAN IP address displays
in the field to the right (read-only). The NBG tells the DHCP clients on the LAN that the NBG
itself is the DNS server. When a computer on the LAN sends a DNS query to the NBG, the
NBG forwards the query to the NBG's system DNS server (configured in the
WAN >
Internet Connection
screen) and relays the response back to the computer. You can only
select
DNS Relay
for one of the three servers; if you select
DNS Relay
for a second or
third DNS server, that choice changes to
None
after you click
Apply
.
Select
None
if you do not want to configure DNS servers. If you do not configure a DNS
server, you must know the IP address of a computer in order to access it.
Apply
Click
Apply
to save your changes back to the NBG.
Reset
Click
Reset
to begin configuring this screen afresh.
Table 57
Network > DHCP Server > Advanced (continued)
LABEL
DESCRIPTION
Page 117 / 249
NBG6515 User’s Guide
117
C
HAPTER
16
Network Address Translation (NAT)
16.1
Overview
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in
a packet. For example, the source address of an outgoing packet, used within one network is
changed to a different IP address known within another network.
Each packet has two addresses – a source address and a destination address. For outgoing packets,
NAT maps private (local) IP addresses to globally unique ones required for communication with
hosts on other networks. It replaces the original IP source address in each packet and then
forwards it to the Internet. The NBG keeps track of the original addresses and port numbers so
incoming reply packets can have their original values restored. The following figure illustrates this.
Figure 88
NAT Example
For more information on IP address translation, refer to
RFC 1631
,
The IP Network Address
Translator (NAT)
.
16.2
What You Can Do
•
Use the
General
screen (
Section 16.3 on page 118
) to enable NAT and set a default server.
•
Use the
Application
screen (
Section 16.4 on page 118
) o forward incoming service requests to
the server(s) on your local network.
•
Use the
Advanced
screen (
Section 16.5 on page 120
) to change your NBG’s trigger port
settings.
Page 118 / 249
Chapter 16 Network Address Translation (NAT)
NBG6515 User’s Guide
118
16.3
General NAT Screen
Use this screen to enable NAT and set a default server. Click
Network > NAT > General
to open
the following screen.
Figure 89
Network > NAT > General
The following table describes the labels in this screen.
16.4
NAT Application Screen
Use the
Application
screen to forward incoming service requests to the server(s) on your local
network. You may enter a single port number or a range of port numbers to be forwarded, and the
local IP address of the desired server. The port number identifies a service; for example, web
service is on port 80 and FTP on port 21. In some cases, such as for unknown services or where one
server can support more than one service (for example both FTP and web service), it might be
better to specify a range of port numbers.
In addition to the servers for specified services, NAT supports a default server. A service request
that does not have a server explicitly designated for it is forwarded to the default server. If the
default is not defined, the service request is simply discarded.
Table 58
Network > NAT > General
LABEL
DESCRIPTION
NAT Setup
Enable Network
Address
Translation
Network Address Translation (NAT) allows the translation of an Internet protocol address
used within one network (for example a private IP address used in a local network) to a
different IP address known within another network (for example a public IP address used on
the Internet).
Select the check box to enable NAT.
Default Server Setup
Server IP
Address
In addition to the servers for specified services, NAT supports a default server. A default
server receives packets from ports that are not specified in the
Application
screen.
If you do not assign a
Default
Server
IP address
, the NBG discards all packets received
for ports that are not specified in the
Application
screen or remote management.
Apply
Click
Apply
to save your changes back to the NBG.
Reset
Click
Reset
to begin configuring this screen afresh.
Page 119 / 249
Chapter 16 Network Address Translation (NAT)
NBG6515 User’s Guide
119
Note: Many residential broadband ISP accounts do not allow you to run any server
processes (such as a Web or FTP server) from your location. Your ISP may
periodically check for servers and may suspend your account if it discovers any
active services at your location. If you are unsure, refer to your ISP.
Port forwarding allows you to define the local servers to which the incoming services will be
forwarded. To change your NBG’s port forwarding settings, click
Network > NAT
>
Application
.
The screen appears as shown.
Note: If you do not assign a
Default Server
IP address
in the
NAT >
General
screen,
the NBG discards all packets received for ports that are not specified in this screen
or remote management.
Refer to
Appendix D on page 225
for port numbers commonly used for particular services.
Figure 90
Network > NAT > Application
The following table describes the labels in this screen.
Table 59
Network > NAT > Application
LABEL
DESCRIPTION
Add Application Rule
Active
Select the check box to enable this rule and the requested service can be forwarded to the
host with a specified internal IP address.
Clear the checkbox to disallow forwarding of these ports to an inside server without having
to delete the entry.
Service Name
Type a name (of up to 31 printable characters) to identify this rule in the first field next to
Service Name
. Otherwise, select a predefined service in the second field next to
Service
Name
. The predefined service name and port number(s) will display in the
Service Name
and
Port
fields.
Port
Type a port number(s) to define the service to be forwarded to the specified server.
To specify a range of ports, enter a hyphen (-) between the first port and the last port, such
as 10-20.
To specify two or more non-consecutive port numbers, separate them by a comma without
spaces, such as 123,567.
Server IP
Address
Type the IP address of the server on your LAN that receives packets from the port(s)
specified in the
Port
field.
Page 120 / 249
Chapter 16 Network Address Translation (NAT)
NBG6515 User’s Guide
120
16.5
NAT Advanced Screen
Some services use a dedicated range of ports on the client side and a dedicated range of ports on
the server side. With regular port forwarding you set a forwarding port in NAT to forward a service
(coming in from the server on the WAN) to the IP address of a computer on the client side (LAN).
The problem is that port forwarding only forwards a service to a single LAN IP address. In order to
use the same service on a different LAN computer, you have to manually replace the LAN
computer's IP address in the forwarding port with another LAN computer's IP address.
Trigger port forwarding solves this problem by allowing computers on the LAN to dynamically take
turns using the service. The NBG records the IP address of a LAN computer that sends traffic to the
WAN to request a service with a specific port number and protocol (a "trigger" port). When the
NBG's WAN port receives a response with a specific port number and protocol ("incoming" port),
the NBG forwards the traffic to the LAN IP address of the computer that sent the request. After that
computer’s connection for that service closes, another computer on the LAN can use the service in
the same manner. This way you do not need to configure a new IP address each time you want a
different LAN computer to use the application.
To change your NBG’s trigger port settings, click
Network > NAT
>
Advanced
. The screen
appears as shown.
Note: Only one LAN computer can use a trigger port (range) at a time.
Application Rules Summary
#
This is the number of an individual port forwarding server entry.
Active
This icon is turned on when the rule is enabled.
Name
This field displays a name to identify this rule.
Port
This field displays the port number(s).
Server IP
Address
This field displays the inside IP address of the server.
Modify
Click the
Edit
icon to display and modify an existing rule setting in the fields under
Add
Application Rule
.
Click the
Remove
icon to delete a rule.
Apply
Click
Apply
to save your changes back to the NBG.
Reset
Click
Reset
to begin configuring this screen afresh.
Table 59
Network > NAT > Application (continued)
LABEL
DESCRIPTION

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