Cisco Systems 78-11741-02 Wireless Office Headset User Manual


 
Configuring IP Addressing
Configuring Network Address Translation
IPC-35
Cisco IOS IP Configuration Guide
Configuring Network Address Translation
Two key problems facing the Internet are depletion of IP address space and scaling in routing. Network
Address Translation (NAT) is a feature that allows the IP network of an organization to appear from the
outside to use different IP address space than what it is actually using. Thus, NAT allows an organization
with nonglobally routable addresses to connect to the Internet by translating those addresses into
globally routable address space. NAT also allows a more graceful renumbering strategy for organizations
that are changing service providers or voluntarily renumbering into classless interdomain routing
(CIDR) blocks. NAT is also described in RFC 1631.
Beginning with Cisco IOS Release 12.1(5)T, NAT supports all H.225 and H.245 message types,
including FastConnect and Alerting as part of the H.323 version 2 specification. Any product that makes
use of these message types will be able to pass through a Cisco IOS NAT configuration without any static
configuration. Full support for NetMeeting Directory (Internet Locator Service) is also provided through
Cisco IOS NAT.
NAT Applications
NAT has several applications. Use it for the following purposes:
You want to connect to the Internet, but not all your hosts have globally unique IP addresses. NAT
enables private IP internetworks that use nonregistered IP addresses to connect to the Internet. NAT
is configured on the router at the border of a stub domain (referred to as the inside network) and a
public network such as the Internet (referred to as the outside network). NAT translates the internal
local addresses to globally unique IP addresses before sending packets to the outside network.
You must change your internal addresses. Instead of changing them, which can be a considerable
amount of work, you can translate them by using NAT.
You want to do basic load sharing of TCP traffic. You can map a single global IP address to many
local IP addresses by using the TCP load distribution feature.
As a solution to the connectivity problem, NAT is practical only when relatively few hosts in a stub
domain communicate outside of the domain at the same time. When this is the case, only a small subset
of the IP addresses in the domain must be translated into globally unique IP addresses when outside
communication is necessary, and these addresses can be reused when no longer in use.
Benefits
A significant advantage of NAT is that it can be configured without requiring changes to hosts or routers
other than those few routers on which NAT will be configured. As discussed previously, NAT may not
be practical if large numbers of hosts in the stub domain communicate outside of the domain.
Furthermore, some applications use embedded IP addresses in such a way that it is impractical for a NAT
device to translate. These applications may not work transparently or at all through a NAT device. NAT
also hides the identity of hosts, which may be an advantage or a disadvantage.
A router configured with NAT will have at least one interface to the inside and one to the outside. In a
typical environment, NAT is configured at the exit router between a stub domain and backbone. When a
packet is leaving the domain, NAT translates the locally significant source address into a globally unique
address. When a packet is entering the domain, NAT translates the globally unique destination address
into a local address. If more than one exit point exists, each NAT must have the same translation table.
If the software cannot allocate an address because it has run out of addresses, it drops the packet and
sends an ICMP host unreachable packet.