User's Manual

396
Users Manual of CS-6306R
interface vlan 11
ip address 202.96.2.3 255.255.255.0
Configuring NAT 43.2
43.2.1 Introduction
The Internet faces two key problems: insufficient IP address space and route measurement. Network
Address Translation (NAT) is an attribute. You can find that a group of IP networks with this attribute use
different IP address spaces, but you cannot find the actual address space used by the group of networks. By
transforming these addresses to the address spaces that can be globally routed, NAT permits an organization
without global routing addresses to connect the Internet. NAT also permits good recoding strategy to change
the service providers for the organizations or to automatically code to the CIDR module. NAT will be described
in RFC 1631.
43.2.1.1 NAT Application
Main NAT applications are shown as follows:
 All hosts need to connect to the Internet, but no all hosts have a unique global IP address. NAT enables
unregistered networks with private IP addresses to connect the Internet. NAT are always configured at
the routing switch between inside network and Internet. Before sending message to the Internet, NAT
transfers the inside local address to the unique global IP address.
 The inside address has to be modified. You can transform the address by using NAT without too much
time.
 The basic TCP transmission load balance need be realized. You can map a single global IP address to
multiple IP addresses using TCP load distribution characteristic.
 As a resolution for connection problems, NAT can be used when relatively few hosts in an inside
network communicate with the Internet. In this case, the IP addresses of few hosts will be transformed to
a unique global IP address when they communicate with the Internet. These addresses can be reused
when they are not used any more.
43.2.1.2 NAT Advantage
An obvious advantage of NAT is that you can perform configuration without modifying host or switch. As said
above, NAT is useless if many hosts in a single-connection domain communicate with the outside. What’s
more, the NAT device is not suitable to translate the embedded IP address. These applications cannot work
transparently or completely (without translation) pass through a NAT device. NAT hides the identifier of the
host, which may be an advantage or a shortcoming.
The router configured with NAT has at least one inside interface and one outside interface. In typical case,
NAT is configured at the router between the single-connection domain and the backbone domain. When a
message is leaving the single-connection domain, NAT transforms the effective local address to a unique
global address. When the message reaches the domain, NAT transforms the unique global address to the
local address. If multiple interfaces exist, each NAT must have the same the transfer table. If no address is