Napt - H3C S9500 Series Operation Manual

Operation Manual – NAT-URPF-VPLS
H3C S9500 Series Routing Switches
Internet access authority, and refuses to perform address translation for those ineligible.
In a word, this involves NAT control.
Many-to-many NAT can be implemented by defining an address pool, and the control of
NAT can be achieved by employing access control lists (ACLs).
An address pool is a collection of public IP addresses for NAT. Its configuration
depends on the number of available public IP addresses, the number of internal hosts,
and the practical application. During address translation, the NAT process selects an
address from the address pool to use as the translated source address.
The control of NAT is implemented by using ACLs. Only packets matching the ACL
criteria can obtain NAT service. In this way, the application of NAT is effectively
controlled, making only some specific hosts have the authority to access the Internet.

1.2.2 NAPT

NAT maps an Internal IP address to an external IP address and creates a
corresponding entry in the address mapping table, and the external IP address is then
unavailable to any hosts other than the one currently using it if the address mapping
entry does not get purged. Different from NAT, NAPT (network address port translation)
is capable of mapping an IP address to multiple IP addresses or mapping a group of IP
addresses to another group of IP addresses. That is, NAPT saves external address
resource more efficiently. NAPT is also known as PAT (Port Address Translation) or
Address Overloading.
NAPT involves the mapping of IP addresses and transport layer protocol port numbers.
NAPT maps multiple sessions, each of which has its own source address, to different
ports of the same external address, through which multiple internal addresses can
multiplex one or more external addresses.
The following figure illustrates the fundamentals of NAPT.
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Chapter 1 NAT Configuration