Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
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11.8.3 How NAT Works
Each packet has tw o addresses – a source address and a destination address. For out going packets,
the I LA (I nside Local Address) is the source address on t he LAN, and the I GA (I nside Global
Address) is the source address on t he WAN. For incom ing packet s, t he I LA is t he destination
address on t he LAN, and the I GA is the dest ination address on the WAN. NAT m aps private ( local)
I P addresses t o globally unique ones r equir ed for com m unication with host s on ot her net works. I t
replaces the original I P source address (and TCP or UDP source port num bers for Many-t o- One and
Many- to- Many Overload NAT m apping) in each packet and t hen forwards it t o the I nt ernet. The
Device keeps track of the original addresses and port num bers so incom ing reply packets can have
their original values restored. The following figure illustrates t his.
Figure 84 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)
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