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Chapter 10 Network Address Translation (NAT) Screens
10.1.2 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 – see
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)
.
10.1.3 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
and port numbers so incoming reply packets can have their original values restored. The
following figure illustrates this.
Figure 75
How NAT Works
Summary of Contents for P-2608HWL-D1
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Page 32: ...P 2608HWL Dx Series User s Guide 32 List of Figures ...
Page 38: ...P 2608HWL Dx Series User s Guide 38 List of Tables ...
Page 44: ...P 2608HWL Dx Series User s Guide 44 Chapter 1 Getting To Know the ZyXEL Device ...
Page 66: ...P 2608HWL Dx Series User s Guide 66 Chapter 3 Internet and Wireless Setup Wizard ...
Page 72: ...P 2608HWL Dx Series User s Guide 72 Chapter 4 VoIP Wizard And Example ...
Page 78: ...P 2608HWL Dx Series User s Guide 78 Chapter 5 Bandwidth Management Wizard ...
Page 88: ...P 2608HWL Dx Series User s Guide 88 Chapter 6 Status Screens ...
Page 118: ...P 2608HWL Dx Series User s Guide 118 Chapter 8 LAN Setup ...
Page 138: ...P 2608HWL Dx Series User s Guide 138 Chapter 9 Wireless LAN ...
Page 166: ...P 2608HWL Dx Series User s Guide 166 Chapter 11 SIP ...
Page 176: ...P 2608HWL Dx Series User s Guide 176 Chapter 12 Phone ...
Page 184: ...P 2608HWL Dx Series User s Guide 184 Chapter 13 Phone Book ...
Page 220: ...P 2608HWL Dx Series User s Guide 220 Chapter 17 Content Filtering ...
Page 276: ...P 2608HWL Dx Series User s Guide 276 Chapter 20 Static Route ...
Page 288: ...P 2608HWL Dx Series User s Guide 288 Chapter 21 Bandwidth Management ...
Page 292: ...P 2608HWL Dx Series User s Guide 292 Chapter 22 Dynamic DNS Setup ...
Page 306: ...P 2608HWL Dx Series User s Guide 306 Chapter 23 Remote Management Configuration ...
Page 318: ...P 2608HWL Dx Series User s Guide 318 Chapter 24 Universal Plug and Play UPnP ...
Page 324: ...P 2608HWL Dx Series User s Guide 324 Chapter 25 System ...
Page 344: ...P 2608HWL Dx Series User s Guide 344 Chapter 27 Tools ...
Page 348: ...P 2608HWL Dx Series User s Guide 348 Chapter 28 Diagnostic ...
Page 360: ...P 2608HWL Dx Series User s Guide 360 Chapter 29 Troubleshooting ...
Page 378: ...P 2608HWL Dx Series User s Guide 378 Appendix B Setting up Your Computer s IP Address ...
Page 402: ...P 2608HWL Dx Series User s Guide 402 Appendix F Triangle Route ...
Page 440: ...P 2608HWL Dx Series User s Guide 440 Appendix H Internal SPTGEN ...