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Chapter 10 Quality of Service (QoS)
P-2612HNU-Fx User’s Guide
201
DSCP is backward compatible with the three precedence bits in the ToS octet so
that non-DiffServ compliant, ToS-enabled network device will not conflict with the
DSCP mapping.
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior),
that each packet gets across the DiffServ network. Based on the marking rule,
different kinds of traffic can be marked for different kinds of forwarding. Resources
can then be allocated according to the DSCP values and the configured policies.
DSCP (6 bits)
Unused (2 bits)
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C
HAPTER
11
Network Address Translation
(NAT)
11.1
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 to a different IP address known within another
network.
11.1.1
What You Can Do in this Chapter
•
Use the
Port Forwarding
screen to configure forward incoming service
requests to the server(s) on your local network (
Section 11.2 on page 204
).
•
Use the
Sessions
screen to limit the number of concurrent NAT sessions each
client can use (
Section 11.3 on page 207
).
11.1.2
What You Need To Know
The following terms and concepts may help as you read this chapter.
Inside/Outside and Global/Local
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.
NAT
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)
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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.
Port Forwarding
A port forwarding set is a list of inside (behind NAT on the LAN) servers, for
example, web or FTP, that you can make visible to the outside world even though
NAT makes your whole inside network appear as a single computer to the outside
world.
Finding Out More
See
Section 11.4 on page 208
for advanced technical information on NAT.
11.2
The Port Forwarding Screen
Use the
Port Forwarding
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. You can allocate a server IP address that corresponds to
a port or a range of ports.
The most often used port numbers and services are shown in
Appendix E on page
387
. Please refer to RFC 1700 for further information about port numbers.
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.
Configuring Servers Behind Port Forwarding (Example)
Let's say you want to assign ports 21-25 to one FTP, Telnet and SMTP server (
A
in
the example), port 80 to another (
B
in the example) and assign a default server IP
address of 192.168.1.35 to a third (
C
in the example). You assign the LAN IP
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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 387
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 45
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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