H3C XE 200/2000 IP, Руководство по эксплуатации

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Operation Manual – Location Server Configuration
H3C XE 200/2000 IP PBX
Chapter 2 NAT/FW Tunnel Traversal
2-1
Chapter 2 NAT/FW Tunnel Traversal
2.1 Overview
2.1.1 Introduction to NAT
Network address translation (NAT) is a technology that translates private network
addresses into valid public network addresses transparently according to specific rules.
The internal addresses used in a private network are invisible to network devices
outside the network and they must be translated into valid public network addresses by
NAT for hosts using these private network addresses to communicate with hosts in
public networks. This sort of address translation is implemented through the address
mapping table maintained by the NAT device. NAT helps to save the IP address
resources. It also isolates private networks from public networks so that the private
network topologies are concealed from outside and the private networks are secured in
certain degree.
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The following describes the principle of address translation for NAT traversal from
private network to public network:
Private Network Public Network
Source IP : Private network IP address
Destination IP : Public network address
Source IP : Public network IP address
Destination IP : Public network address
NAT/FW
IP Package IP Package
Figure 2-1 Diagram for address translation for NAT traversal from private network to
public network
When the NAT device receives an IP packet from the private network, it translates and
changes the source IP address of the packet (a private network address) to a public
network IP address it owns, updates the address mapping table on itself to reflect the
mapping relationship between the two IP addresses in an entry for the use of the
reverse translation from the public address to the private address, and then sends the
packet to the destination public network. In this procedure, if the NAT device fails to
translate the source address, it discards the packet.
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The following describes the principle of address translation for NAT traversal from
public network to private network: