3Com MSR 50 Series Configuration Manual page 1814

1814 C 96: NAT C
HAPTER ONFIGURATION
Figure 527 An NAPT process
Host
192 . 168 . 1 . 3
Server A
192 . 168 . 1 . 2
As illustrated in the above figure, four data packets arrive at the NAT gateway.
Packets 1 and 2 have the same internal address but different source port numbers.
Packets 3 and 4 have different internal addresses but the same source port
number. NAPT maps the four data packets to the same external address but with
different source port numbers. Therefore, the packets can still be discriminated.
When response packets arrive, the NAT gateway can forward them to the
corresponding hosts based on the destination address and port numbers.
Internal server
NAT hides the internal network structure, including the identities of internal hosts.
However, in practice, external contacts to internal hosts are sometimes also
necessary. In this case, you need an internal server, such as a WWW server or an
FTP server to provide such services. With NAT, you can deploy an internal server
easily and flexibly. For instance, you can use 20.1.1.10 as the WWW server's
external address, 20.1.1.11 as the FTP server's external address; or you can even
use such address 20.1.1.12:8080 as the WWW server's external address.
Currently, this feature is available on the device. When an external user accesses
an internal server, NAT translates the destination address in the request packet to
the private IP address of the internal server. When the internal server returns a
packet, NAT translates the source address (a private IP address) of the packet into a
public IP address.
Easy IP
Easy IP allows the NAT gateway to use the public IP address of an interface as the
translated source address for NAT. Besides, the NAT gateway can also use ACLs to
define the internal IP addresses for NAT.
Support for special protocols
Apart from the basic address translation function, NAT also provides a perfect
application layer gateway mechanism that supports various special application
protocols without modifying the NAT platform. Because of this, NAT offers high
scalability. These special protocols include: Internet control message protocol
(ICMP), domain name system (DNS), Internet locator service (ILS), real-time
IP packet 1
Source IP : 192 . 168 . 1 . 3
Source port : 1537
IP packet 2
Source IP : 192 . 168 . 1 . 3
Source port : 2468
192 . 168 . 1 . 1 20 . 1 . 1 . 1
IP packet 3
Source IP : 192 . 168 . 1 . 1
Source port : 1111
IP packet 4
Source IP : 192 . 168 . 1 . 2
Source port : 1111
IP packet 1
Source IP : 20 . 1 . 1 . 1
Source port : 2001
IP packet 2
Source IP : 20 . 1 . 1 . 1
Source port : 2002
Internet
IP packet 3
Source IP : 20 . 1 . 1 . 1
Source port : 2003
IP packet 4
Source IP : 20 . 1 . 1 . 1
Source port : 2004
Server B
10 . 1 . 1 . 2
Host
10 . 1 . 1 . 3