How Nat Works; Nat Application - ZyXEL Communications SBG5500-A User Manual

8.7.3 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 SBG 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 105 How NAT Works

8.7.4 NAT Application

The following figure illustrates a possible NAT application, where three inside LANs (logical LANs using IP
alias) behind the SBG can communicate with three distinct WAN networks.
Chapter 8 Network Address Translation (NAT)
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