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Page 191 / 367 Scroll up to view Page 186 - 190
Chapter 11 Network Address Translation (NAT)
VMG4380-B10A / VMG4325-B10A User’s Guide
191
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.
NAT never changes the IP address (either local or global) of an outside host.
11.8.2
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 (for Many-
to-One and Many-to-Many Overload mapping), NAT offers the additional benefit of firewall
protection. With no servers defined, your 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)
.
Table 64
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 192 / 367
Chapter 11 Network Address Translation (NAT)
VMG4380-B10A / VMG4325-B10A User’s Guide
192
11.8.3
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
Device 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 87
How NAT Works
192.168.1.13
192.168.1.10
192.168.1.11
192.168.1.12
SA
192.168.1.10
SA
IGA1
Inside Local
IP Address
192.168.1.10
192.168.1.11
192.168.1.12
192.168.1.13
Inside Global
IP Address
IGA 1
IGA 2
IGA 3
IGA 4
NAT Table
WAN
LAN
Inside Local
Address (ILA)
Inside Global
Address (IGA)
Page 193 / 367
Chapter 11 Network Address Translation (NAT)
VMG4380-B10A / VMG4325-B10A User’s Guide
193
11.8.4
NAT Application
The following figure illustrates a possible NAT application, where three inside LANs (logical LANs
using IP alias) behind the Device can communicate with three distinct WAN networks.
Figure 88
NAT Application With IP Alias
Port Forwarding: Services and Port Numbers
The most often used port numbers are shown in the following table. Please refer to RFC 1700 for
further information about port numbers. Please also refer to the Supporting CD for more examples
and details on port forwarding and NAT.
Table 65
Services and Port Numbers
SERVICES
PORT NUMBER
ECHO
7
FTP (File Transfer Protocol)
21
SMTP (Simple Mail Transfer Protocol)
25
DNS (Domain Name System)
53
Finger
79
HTTP (Hyper Text Transfer protocol or WWW, Web)
80
POP3 (Post Office Protocol)
110
NNTP (Network News Transport Protocol)
119
SNMP (Simple Network Management Protocol)
161
SNMP trap
162
PPTP (Point-to-Point Tunneling Protocol)
1723
Page 194 / 367
Chapter 11 Network Address Translation (NAT)
VMG4380-B10A / VMG4325-B10A User’s Guide
194
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 addresses and the ISP assigns the WAN IP address.
The NAT network appears as a single host on the Internet.
Figure 89
Multiple Servers Behind NAT Example
D=192.168.1.36
192.168.1.1
IP address assigned by ISP
A=192.168.1.33
B=192.168.1.34
C=192.168.1.35
Page 195 / 367
VMG4380-B10A / VMG4325-B10A User’s Guide
195
C
HAPTER
12
Dynamic DNS Setup
12.1
Overview
DNS
DNS (Domain Name System) is for mapping a domain name to its corresponding IP address and
vice versa. The DNS server is extremely important because without it, you must know the IP
address of a machine before you can access it.
In addition to the system DNS server(s), each WAN interface (service) is set to have its own static
or dynamic DNS server list. You can configure a DNS static route to forward DNS queries for certain
domain names through a specific WAN interface to its DNS server(s). The Device uses a system
DNS server (in the order you specify in the
Broadband
screen) to resolve domain names that do
not match any DNS routing entry. After the Device receives a DNS reply from a DNS server, it
creates a new entry for the resolved IP address in the routing table.
Dynamic DNS
Dynamic DNS allows you to update your current dynamic IP address with one or many dynamic
DNS services so that anyone can contact you (in NetMeeting, CU-SeeMe, etc.). You can also access
your FTP server or Web site on your own computer using a domain name (for instance
myhost.dhs.org, where myhost is a name of your choice) that will never change instead of using an
IP address that changes each time you reconnect. Your friends or relatives will always be able to
call you even if they don't know your IP address.
First of all, you need to have registered a dynamic DNS account with www.dyndns.org. This is for
people with a dynamic IP from their ISP or DHCP server that would still like to have a domain name.
The Dynamic DNS service provider will give you a password or key.
12.1.1
What You Can Do in this Chapter
•
Use the
DNS Entry
screen to view, configure, or remove DNS routes (
Section 12.2 on page
196
).
•
Use the
Dynamic DNS
screen to enable DDNS and configure the DDNS settings on the Device
(
Section 12.3 on page 197
).

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