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Page 106 / 237 Scroll up to view Page 101 - 105
Chapter 10 Network Address Translation
NBG-418N v2 User’s Guide
106
The global IP addresses for the inside hosts can be either static or dynamically assigned by the ISP.
In addition, you can designate servers, for example, a web server and a telnet server, on your local
network and make them accessible to the outside world. If you do not define any servers, NAT
offers the additional benefit of firewall protection. With no servers defined, your NBG-418N v2
filters out all incoming inquiries, thus preventing intruders from probing your network. For more
information on IP address translation, refer to
RFC 1631
,
The IP Network Address Translator (NAT)
.
How NAT Works
Each packet has two addresses – a source address and a destination address. For outgoing packets,
the ILA (Inside Local Address) is the source address on the LAN, and the IGA (Inside Global
Address) is the source address on the WAN. For incoming packets, the ILA is the destination
address on the LAN, and the IGA is the destination address on the WAN. 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-418N v2 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 75
How NAT Works
10.3
General NAT Screen
Use this screen to enable NAT and set a default server. Click
Network > NAT
to open the
General
screen.
Figure 76
Network > NAT > General
Page 107 / 237
Chapter 10 Network Address Translation
NBG-418N v2 User’s Guide
107
The following table describes the labels in this screen.
10.4
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.
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-418N v2’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-418N v2 discards all packets received for ports that are not specified in
this screen or remote management.
Refer to
Appendix E on page 217
for port numbers commonly used for particular services.
Table 48
Network > NAT > General
LABEL
DESCRIPTION
NAT Setup
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).
This field displays whether the NAT is enabled or disabled.
Default Server Setup
Enable
Click the
Enable
check box to activate the default server.
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-418N v2 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-418N v2.
Reset
Click
Reset
to begin configuring this screen afresh.
Page 108 / 237
Chapter 10 Network Address Translation
NBG-418N v2 User’s Guide
108
Figure 77
Network > NAT > Application
The following table describes the labels in this screen.
10.5
Port Triggering Screen
To change your NBG-418N v2’s port trigger settings, click
Network > NAT > Port Triggering
.
The screen appears as shown.
Table 49
Network > NAT > Application
LABEL
DESCRIPTION
Add Application Rule
Application Name
Select an option from the drop-down list to choose a pre-defined service. The pre-defined
service port number(s) and protocol will display in the fields below.
User-Defined
Application Name
Type a name (of up to 31 printable characters) to identify this rule. Otherwise, select a
predefined service in the
Application Name
drop-down list.
Protocol
Select the transport layer protocol used for the service. Choices are
TCP
, and
UDP
.
Public Port Range
Local Port Range
Type a port number(s) to be forwarded.
To specify a range of ports, enter a colon (:) between the first port and the last port, such
as 10:20.
Server IP Address
Type the inside IP address of the server that receives packets from the port(s) specified in
the
Port
field.
Apply
Click
Apply
to save your changes to the
Application Rules Summary
table.
Reset
Click
Reset
to not save and return your new changes in the
Service Name
and
Port
fields to the previous one.
Application Rules Summary
Application Name
This field displays a name to identify this rule.
Protocol
This field displays the transport layer protocol supported by this server.
Server IP Address
This field displays the inside IP address of the server.
Local Port Range
Public Port Range
This field displays the port number(s).
State
This field displays whether the rule is enabled or disabled.
Action
Click the
Disable
icon to disable a rule.
Click the
Delete
icon to delete a rule.
Page 109 / 237
Chapter 10 Network Address Translation
NBG-418N v2 User’s Guide
109
Note: Only one LAN computer can use a port trigger (range) at a time.
Figure 78
Network > NAT > Port Triggering
The following table describes the labels in this screen.
Table 50
Network > NAT > Port Triggering
LABEL
DESCRIPTION
Port Triggering Status
Nat Port Trigger
Click
Enable
radio button to enable NAT Port Trigger or
Disable
to inactivate it.
Apply
Click
Apply
button to apply the NAT Port Trigger status you choose above.
Add Application Rule
User-defined
Application Name
Type a unique name (up to 15 characters) for identification purposes. All characters are
permitted - including spaces.
Start Match
Port
Enter the starting port in a range of port numbers that causes (or triggers) the NBG-418N
v2 to record the IP address of the LAN computer that sent the traffic to a server on the
WAN.
End Match
Port
Enter the ending port in a range of port numbers that causes (or triggers) the NBG-418N
v2 to record the IP address of the LAN computer that sent the traffic to a server on the
WAN.
Trigger
Protocol
Specify the protocol (
UDP
,
TCP
or
UDP/TCP
) that causes (or triggers) the NBG-418N v2
to record the IP address of the LAN computer that sent the traffic to a server on the WAN.
Start Related
Port
Enter the starting port in a range of port numbers that a server on the WAN uses when it
sends out a particular service. The NBG-418N v2 forwards the traffic with this starting
port to the client computer on the LAN that requested the service.
End Related
Port
Enter the ending port in a range of port numbers that a server on the WAN uses when it
sends out a particular service. The NBG-418N v2 forwards the traffic with this ending port
to the client computer on the LAN that requested the service.
Page 110 / 237
Chapter 10 Network Address Translation
NBG-418N v2 User’s Guide
110
10.6
Technical Reference
The following section contains additional technical information about the NBG-418N v2 features
described in this chapter.
10.6.1
NAT Port Forwarding: Services and Port Numbers
A port forwarding set is a list of inside (behind NAT on the LAN) servers, for example, web or FTP,
that you can make accessible to the outside world even though NAT makes your whole inside
network appear as a single machine to the outside world.
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.
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.
Open Protocol
Specify the protocol (
UDP
,
TCP
or
UDP/TCP
) that a server on the WAN uses when it
sends out a particular service.
Apply
Click
Apply
to save your changes back to the NBG-418N v2.
Reset
Click
Reset
to begin configuring this screen afresh.
Application Rules Summary
Service Name
This field displays the name of the application rule.
Trigger Protocol
This field displays the protocol that causes (or triggers) the NBG-418N v2 to record the IP
address of the LAN computer that sent the traffic to a server on the WAN.
Direction
This field displays the direction of travel of packets that match this rule.
Match Port
This field displays the port(s) that causes (or triggers) the NBG-418N v2 to record the IP
address of the LAN computer that sent the traffic to a server on the WAN.
Open Protocol
This field displays the protocol a server on the WAN uses when it sends out a particular
service.
Related Port
This field displays the port(s) a server on the WAN uses when it sends out a particular
service. The EMG2926-Q10A forwards the traffic with this port (or range of ports) to the
client computer on the LAN that requested the service.
Action
Click
Delete
to remove the rule.
Table 50
Network > NAT > Port Triggering (continued)
LABEL
DESCRIPTION

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