How Nat Works - ZyXEL Communications VMG1312-B Series User Manual

Chapter 11 Network Address Translation (NAT)

11.8.3 How NAT Works

Each packet has t wo addresses – a source address and a dest inat ion address. For out going packet s,
t he I LA ( I nside Local Address) is t he source address on t he LAN, and t he I GA ( I nside Global
Address) is t he source address on t he WAN. For incom ing packet s, t he I LA is t he dest inat ion
address on t he LAN, and t he I GA is t he dest inat ion address on t he WAN. NAT m aps privat e ( local)
I P addresses t o globally unique ones required for com m unicat ion wit h host s on ot her net works. I t
replaces t he original I P source address ( and TCP or UDP source port num bers for Many- t o- One and
Many- t o- Many Overload NAT m apping) in each packet and t hen forwards it t o t he I nt ernet . The
Device keeps t rack of t he original addresses and port num bers so incom ing reply packet s can have
t heir original values rest ored. The following figure illust rat es t his.
Figure 84 How NAT Works
LAN
192.168.1.12
192.168.1.11
198
192.168.1.13
SA
192.168.1.10
Inside Local
Address (ILA)
192.168.1.10
NAT Table
Inside Local Inside Global
IP Address
IP Address
192.168.1.10
IGA 1
192.168.1.11
IGA 2
192.168.1.12
IGA 3
192.168.1.13
IGA 4
SA
IGA1
Inside Global
Address (IGA)
VMG1312-B Series User's Guide
WAN