Figure 7-3 illustrates a router that is translating a source address inside a network into a
source address outside the network.
Figure 7-3 Translating an Address
The steps for translating an inside source address are as follows:
Step 1 The user at host 1.1.1.1 opens a connection to host B.
Step 2 The ?¬?rst packet that the router receives from host 1.1.1.1 causes the
router to check its NAT table.
1.1.1.2
1.1.1.1 1
SA
1.1.1.1
2
5
DA
1.1.1.1
3
SA
2.2.2.2
4
DA
2.2.2.2
Inside Outside
Inside
Interface
Outside
Interface
Inside Global
IPv4 Address
Inside Local
IPv4 Address
2.2.2.3
2.2.2.2
1.1.1.2
1.1.1.1
NAT Table
Internet
Host B
9.6.7.3
254 Chapter 7: Managing Address Spaces with NAT and IPv6
??? If a static translation entry was con?¬?gured, the router goes to Step 3.
??? If no static translation entry exists, the router determines that the source address
1.1.1.1 (SA 1.1.1.1) must be translated dynamically. The router then selects a
legal, global address from the dynamic address pool and creates a translation
entry (in the example, 2.2.2.2). This type of entry is called a simple entry.
Step 3 The router replaces the inside local source address of host 1.
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