NAT operates on a router and usually connects two networks. NAT translates the local nonunique
IP addresses into legal, registered Internet addresses before forwarding packets from the
local network to the Internet or another outside network. To do this, NAT uses a six-step process,
as shown in Figure 31.2.
The six-step process, as Figure 31.2 illustrates, is as follows:
1. User 10.1.2.25 sends a packet and attempts to open a connection to 206.100.29.1.
2. When the first packet arrives at the NAT border router, the router checks to see whether
there is an entry for the local address that matches a global address in the NAT table.
3. If a match is found in the NAT table, the process continues to step 4. If a match is not found,
the NAT router uses what is called a simple entry from its pool of global addresses. A simple
entry occurs when the NAT router matches a local IP address (such as the one currently
being used) to a global IP address. In this example, the NAT router will match the address
of 10.1.2.25 to 200.1.1.25.
4. The NAT border router then replaces the local address of 10.1.2.25 (listed as the packet’s
source address) with 200.1.1.25. This makes the destination host believe that the sending
device’s IP address is 200.1.1.25.
FIGURE 3 1 . 2 The process of translating inside local addresses
1
10.1.2.25
10.1.2.26
10.1.2.27
Inside
network
switch
NAT border
router
Inside IP Inside global IP
206.100.29.1
10.1.2.25 200.1.1.25
6
2 4
3
5
Interne
5. When the host on the Internet using the IP address 206.100.29.1 replies, it uses the NAT
router–assigned IP address of 200.1.1.25 as the destination address.
6. When the NAT border router receives the reply from 206.100.29.1 with the packet destined
for 200.1.1.25, the NAT border router checks its NAT table again. The NAT table
shows that the local address of 10.1.2.25 should receive the packet destined for 200.1.1.25
and replaces the destination address with the internal interface’s IP address.
Steps 2 through 6 are repeated for each individual packet.