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Page 191 / 404 Scroll up to view Page 186 - 190
Chapter 11 Network Address Translation (NAT)
P-661HNU-Fx User’s Guide
191
addresses and the ISP assigns the WAN IP address. The NAT network appears as a
single host on the Internet.
Figure 76
Multiple Servers Behind NAT Example
11.2.1
The Port Forwarding Screen
Click
Network Setting > NAT
to open the
Port Forwarding
screen.
See
Appendix E on page 359
for port numbers commonly used for particular
services.
Figure 77
Network Setting > NAT > Port Forwarding
The following table describes the fields in this screen.
A=192.168.1.33
D=192.168.1.36
C=192.168.1.35
B=192.168.1.34
WAN
LAN
192.168.1.1
IP Address assigned by ISP
Table 43
Network Setting > NAT > Port Forwarding
LABEL
DESCRIPTION
Add new rule
Click this to add a new port forwarding rule.
#
This is the index number of the entry.
Status
This field indicates whether the rule is active or not.
Clear the check box to disable the rule. Select the check box to enable
it.
Service Name
This is the service’s name. This shows
User Defined
if you manually
added a service. You can change this by clicking the edit icon.
WAN Interface
This shows the WAN interface through which the service is forwarded.
Start Port
This is the first external port number that identifies a service.
End Port
This is the last external port number that identifies a service.
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192
11.2.2
The Port Forwarding Edit Screen
This screen lets you create or edit a port forwarding rule. Click
Add new rule
in
the
Port Forwarding
screen or the
Edit
icon next to an existing rule to open the
following screen.
Figure 78
Port Forwarding: Add/Edit
The following table describes the labels in this screen.
Translation
Start Port
This is the first internal port number that identifies a service.
Translation End
Port
This is the last internal port number that identifies a service.
Server IP
Address
This is the server’s IP address.
Protocol
This shows the IP protocol supported by this virtual server, whether it is
TCP
,
UDP
, or
TCP/UDP
.
Modify
Click the
Edit
icon to edit the port forwarding rule.
Click the
Delete
icon to delete an existing port forwarding rule. Note
that subsequent address mapping rules move up by one when you take
this action.
Apply
Click
Apply
to save your changes.
Cancel
Click
Cancel
to restore your previously saved settings.
Table 43
Network Setting > NAT > Port Forwarding
LABEL
DESCRIPTION
Table 44
Port Forwarding: Add/Edit
LABEL
DESCRIPTION
Service Name
Enter a name to identify this rule using keyboard characters (A-Z, a-z, 1-
2 and so on).
WAN Interface
Select the WAN interface through which the service is forwarded.
You must have already configured a WAN connection with NAT enabled.
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Chapter 11 Network Address Translation (NAT)
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193
11.3
The Sessions Screen
Use the
Sessions
screen to limit the number of concurrent NAT sessions each
client can use.
Click
Network Setting > NAT > Sessions
to display the following screen.
Figure 79
Network Setting > NAT > Sessions
Start Port
Enter the original destination port for the packets.
To forward only one port, enter the port number again in the
External
End Port
field.
To forward a series of ports, enter the start port number here and the
end port number in the
External End Port
field.
End Port
Enter the last port of the original destination port range.
To forward only one port, enter the port number in the
External Start
Port
field above and then enter it again in this field.
To forward a series of ports, enter the last port number in a series that
begins with the port number in the
External Start Port
field above.
Translation
Start Port
This shows the port number to which you want the ZyXEL Device to
translate the incoming port. For a range of ports, enter the first number
of the range to which you want the incoming ports translated.
Translation End
Port
This shows the last port of the translated port range.
Server IP
Address
Enter the inside IP address of the virtual server here.
Protocol Type
Select the protocol supported by this virtual server. Choices are
TCP
,
UDP
, or
TCP/UDP
.
Apply
Click
Apply
to save your changes.
Back
Click
Back
to return to the previous screen without saving.
Table 44
Port Forwarding: Add/Edit (continued)
LABEL
DESCRIPTION
Page 194 / 404
Chapter 11 Network Address Translation (NAT)
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194
The following table describes the fields in this screen.
11.4
Technical Reference
This section provides some technical background information about the topics
covered in this chapter.
11.4.1
NAT Definitions
Inside/outside denotes where a host is located relative to the ZyXEL Device, for
example, the computers of your subscribers are the inside hosts, while the web
servers on the Internet are the outside hosts.
Global/local denotes the IP address of a host in a packet as the packet traverses a
router, for example, the local address refers to the IP address of a host when the
packet is in the local network, while the global address refers to the IP address of
the host when the same packet is traveling in the WAN side.
Note that inside/outside refers to the location of a host, while global/local refers to
the IP address of a host used in a packet. Thus, an inside local address (ILA) is the
IP address of an inside host in a packet when the packet is still in the local
network, while an inside global address (IGA) is the IP address of the same inside
host when the packet is on the WAN side. The following table summarizes this
information.
Table 45
Network Setting > NAT > Sessions
LABEL
DESCRIPTION
MAX NAT
Sessions
Use this field to set a common limit to the number of concurrent NAT
sessions each client computer can have.
If only a few clients use peer to peer applications, you can raise this
number to improve their performance. With heavy peer to peer
application use, lower this number to ensure no single client uses too
many of the available NAT sessions.
Apply
Click
Apply
to save your changes.
Cancel
Click
Cancel
to restore your previously saved settings.
Table 46
NAT Definitions
ITEM
DESCRIPTION
Inside
This refers to the host on the LAN.
Outside
This refers to the host on the WAN.
Local
This refers to the packet address (source or destination) as the packet travels
on the LAN.
Global
This refers to the packet address (source or destination) as the packet travels
on the WAN.
Page 195 / 404
Chapter 11 Network Address Translation (NAT)
P-661HNU-Fx User’s Guide
195
NAT never changes the IP address (either local or global) of an outside host.
11.4.2
What NAT Does
In the simplest form, NAT changes the source IP address in a packet received from
a subscriber (the inside local address) to another (the inside global address)
before forwarding the packet to the WAN side. When the response comes back,
NAT translates the destination address (the inside global address) back to the
inside local address before forwarding it to the original inside host. Note that the
IP address (either local or global) of an outside host is never changed.
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 ZyXEL Device 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)
.
11.4.3
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 (and TCP or UDP source port numbers for
Many-to-One and Many-to-Many Overload NAT mapping) in each packet and then
forwards it to the Internet. The ZyXEL Device keeps track of the original addresses

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