NAT Table

The router performs the following process in overloading inside global addresses:

Step 1 The user at host opens a connection to host B. The first packet that the router receives from host causes the router to check its NAT table.

Step 2 If no translation entry exists, the router determines that address must be translated and sets up a translation of inside local address into a legal inside global address. If overloading is enabled and another translation is active, the router reuses the inside global address from that translation and saves enough information to be able to translate back. This type of entry is called an extended entry.

Step 3 The router replaces the inside local source address with the selected inside global address and forwards the packet.

Step 4 Host B receives the packet and responds to host by using the inside global IP address

Step 5 When the router receives the packet with the inside global IP address, the router performs a NAT table lookup. Using the inside global address and port and outside global address and port as a key, the router translates the address back into the inside local address and forwards the packet to host Host receives the packet and continues the conversation. The router performs Steps 2 through 5 for each packet.

Configuring the DHCP Client and PAT

For a router that connects to the Internet where the provider gives you an address via DHCP, such as DSL or cable connectivity, you need to configure the router as a DHCP client and to perform PAT on the inside private address. The first thing you need to do is determine what interface the DHCP client is to be configured on. Figure 5-17 shows the private and public addresses for this example.

In this implementation, you configure the WAN interface (fa0/1) as a DCHP client so that it get its IP address, default gateway, and default routing from the Internet DHCP server. In addition, you enable PAT to translate the internal private addressing to the external public addressing. For this example, you use Security Device Manager (SDM) to configure DHCP.

Figure 5-17 Identifying Inside and Outside Interfaces

Figure 5-17 Identifying Inside and Outside Interfaces

To begin configuring the DHCP client interface, click the Interfaces and Connections tab. Check the Ethernet (PPPoE or Unencapsulated Routing) radio button, and then click the Create New Connection button. This is shown in Figure 5-18.

Figure 5-18 Configuring the Ethernet Interface

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