Napt - H3C S7500 Series Operation Manual

Operation Manual – NAT, Netstream, Policy Routing
H3C S7500 Series Ethernet Switches

1.2.2 NAPT

With normal NAT, after the private address of an internal host is mapped to a public
address, the public address is unavailable to other internal hosts unless the mapping is
removed. This is called one-to-one NAT.
NAPT (network address port translation) is a variation of NAT. It can map multiple
internal addresses to the same public address, thus allowing multiple internal hosts to
use one public address to access external networks simultaneously. This efficiently
saves public addresses.
NAPT mapping involves the mapping of transport layer protocol port number as well as
the mapping of IP address. When mapping different internal addresses to the same
public address, NAPT maps their port numbers to different port numbers. That is, NAPT
implements the translation between <private address + port> and <public address +
port>.
NAPT is also known as PAT (port address translation) or address overloading.
The following figure illustrates the fundamentals of NAPT.
192 .168 .1.3
PC
192 .168.1.2
Figure 1-2 NAPT address multiplexing
As shown in
as the NAT server:
Packets 1 and 2 carry the same internal address but have different source port
numbers.
Packets 3 and 4 carry different internal addresses but have the same source port
number.
Through NAPT mapping, the four packets are translated into the packets that carry the
same public address but different source port numbers which keeps the four packets
being different. So, when the NAT server receives response packets, it can determine
Packet 1：
Source IP:192.168.1.3
Source Port:15370
Packet 2：
Source IP:192.168.1.3
Source Port:24680
192.168.1.1
Packet 3：
Source IP:192.168.1.1
Source Port:17110
Packet 4：
Source IP:192.168.1.2
Source Port:17110
Figure 1-2, four packets with internal addresses arrive at the switch acting
Packet 1：
Source IP:202.169.10.1
Source Port:15370
Packet 1：
Source IP:202.169.10.1
Source Port:24680
202 .169.10.1
Internet
Switch
Packet 3：
Source IP:202.169.10.1
Source Port:17110
Packet 4：
Source IP:202.169.10.1
Source Port:22220
1-4
Chapter 1 NAT Configuration
202 .120 .10.2
Server
PC
202 .120 .10.3