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The Figure illustrates NAT operation when
it is used to translate addresses from inside your network to
destinations outside of your network.
The steps in the following list
correspond to the numbered NAT operation steps in the Figure:
- User at host 10.1.1.1 opens a
connection to outside host B.
- The first packet that the border
router receives from host 10.1.1.1 causes the router to check its
NAT table.
If a translation is found because it has been statically
configured, the router continues to Step 3. If no translation is
found, the router determines that address 10.1.1.1 must be
translated. The router allocates a new address and sets up a
translation of the inside local address 10.1.1.1 to a legal inside
global address from the dynamic address pool. This type of
translation entry is referred to as a simple entry.
- The border router replaces the
10.1.1.1 inside local IP address with the selected inside global
address, 192.168.2.2, and forwards the packet.
- Host B receives the packet and
responds to that node by using the inside global IP address
192.168.2.2.
- When the border router receives the
packet with the inside global IP address, the router performs a
NAT table lookup by using the inside global address as the
reference. The router then translates the address to the 10.1.1.1
inside local address and forwards the packet to 10.1.1.1. Host
10.1.1.1 receives the packet and continues the conversation. For
each packet, the router performs Steps 2 through 5.
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