Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
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Note t hat inside/ outside refers to the location of a host, while global/ local refers t o the I P addr ess
of a host used in a packet. Thus, an inside local address (I LA) is the I P address of an inside host in
a packet when t he packet is still in t he local network, while an inside global address ( I GA) is the I P
address of t he sam e inside host when t he packet is on t he WAN side. The following table
sum m arizes t his inform at ion.
NAT never changes t he I P address ( either local or global) of an out side host.
11.8.2 What NAT Does
I n t he sim plest form , NAT changes the source I P address in a packet received from a subscriber
( t he inside local address) to anot her ( t he inside global address) before forwarding t he packet to the
WAN side. When t he response com es back, NAT translates t he destination address ( t he inside
global address) back t o the inside local address before forwarding it to the original inside host. Note
that the I P address (eit her local or global) of an outside host is never changed.
The global I P addresses for t he inside host s can be eit her static or dynam ically assigned by t he I SP.
I n addit ion, you can designat e servers, for exam ple, a web server and a telnet server, on your local
net work and m ake t hem accessible to t he out side world. I f you do not define any servers (for Many-
to- One and Many-t o- Many Overload m apping), NAT offers the additional benefit of firewall
prot ect ion. Wit h no servers defined, your Device filt ers out all incom ing inquiries, t hus preventing
intruders from probing your network. For m ore inform at ion on I P address t ranslation, refer to RFC
1631, The I P Network Address Translat or ( NAT) .
Table 61 NAT Definit ions
ITEM DESCRIPTION
I nside This refers to t he host on t he LAN.
Out side This r efers t o t he host on the WAN.
Local This refers t o the packet address ( source or destinat ion) as t he packet t ravels on the
LAN.
Global This refers to t he packet address ( source or destinat ion) as the packet t ravels on the
WAN.
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