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16.10 Understanding Dynamic NAT
Understanding Dynamic NAT Section 16: Enabling Internet Connectivity While static NAT provides a permanent mapping between an internal address and a specific public address, dynamic NAT maps private IP addresses to public addresses from a NAT pool of available addresses. Dynamic NAT configuration differs from static NAT, but it also has some similarities. Like static NAT, it requires the configuration to identify each interface as an inside or outside interface. However, rather than creating a static map to a single IP address, a pool of inside global addresses is used. The figure illustrates a router that is translating a source address inside a network into a source address outside the network. The following steps describe what happens when 10.1.1.100 and 10.1.1.101 want to communicate with 188.8.131.52. However, the figure is showing only what happens in regards to IP host 10.1.1.101. 1. The users at hosts 10.1.1.100 and 10.1.1.101 want to open a connection to host B (IP address 184.108.40.206). 2. The first packet that the router receives from host 10.1.1.101 causes the router to check its NAT table. If no static translation entry exists, the router determines that the source address 10.1.1.101 must be translated dynamically. The router then selects a legal global address from the dynamic address pool and creates a translation entry (in this example, 220.127.116.11). This type of entry is called a simple entry. For the second host, 10.1.1.100, the router selects a legal global address from the dynamic address pool and creates a second translation entry (in this example, 18.104.22.168). 3. The router replaces the inside local source address of host 10.1.1.101 with the translated inside global address and forwards the packet. 4. Host B receives the packet and responds to host 22.214.171.124 by using the inside global IPv4 destination address of 126.96.36.199. When host B receives the second packet, it responds to host 188.8.131.52 by using the inside global IPv4 destination address of 184.108.40.206. 5. When the router receives the packet with the inside global IPv4 address of 220.127.116.11, the router performs a NAT table lookup by using the inside global address as a key. The router then translates the address back to the inside local address of host 10.1.1.101 and forwards the packet to host 10.1.1.101. When the router receives the packet with the inside global IPv4 address of 18.104.22.168, the router performs a NAT table lookup by using the inside global address as a key. The router then translates the address back to the inside local address of host 10.1.1.100 and forwards the packet to host 10.1.1.100. 6. Hosts 10.1.1.100 and 10.1.1.101 receive the packets and continue the conversation. The router performs Steps 2 through 5 for each packet. Up Next: Configuring Dynamic NAT