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NO-PAT
NO-PAT translates a private address to a public address. The public address cannot be used by
another internal host until it is released.
NO-PAT supports all IP packets.
PAT
PAT translates multiple private addresses to a single public address by mapping the private address
and source port to the public address and a unique port. PAT supports TCP and UDP packets, and
ICMP request packets.
Figure 59 PAT operation
As shown in Figure 59, PAT translates the source IP addresses of the three packets to the same
public address and translates their port numbers to different port numbers. Upon receiving a
response, PAT translates the destination address and port number of the response, and forwards it
to the target host.
PAT supports the following mappings:
• Endpoint-Independent Mapping—Uses the same IP and port mapping (EIM entry) for
packets from the same source IP and port to any destination. EIM allows external hosts to
initiate connections to the translated IP addresses and ports of internal hosts. It allows internal
hosts behind different NAT gateways to access each other.
• Address and Port-Dependent Mapping—Uses different IP and port mappings for packets
from the same source IP and port to different destination IP addresses and ports. APDM allows
an external host to initiate connections to an internal host only under the condition that the
internal host has previously accessed the external host. It is secure, but it does not allow
internal hosts behind different NAT gateways to access each other.
NAT Server
The NAT Server feature maps a public address and port number to the private IP address and port
number of an internal server. This feature allows servers in the private network to provide services
for external users.
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