Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 297

on the SNMP agent. These commands use UDP port 161. Some devices can be configured to
send a message to the NMS in the case of an interface failure or other defined event. The
message sent to the server is a SNMP trap, which uses UDP port 162.

NAT

NAT devices convert internal IP address space into globally unique IP addresses. NAT is specified
by RFC 1631. As shown in Figure 6-15, for outgoing IP packets, the source addresses are
converted to globally unique IP addresses. The conversion can be statically done, or you can
use a global pool of addresses dynamically.

Network Address Translation (NAT)

Figure 6-15

Internal Network Internet

NAT Router

Source IP: 192.168.10.100
Destination IP: 210.100.20.1

Source IP: 200.100.10.100
Destination IP: 210.100.20.1

The several forms of NAT follow:
•

Static NAT —Maps an unregistered IP address to a registered IP address, configured
manually.

•

Dynamic NAT —Dynamically maps an unregistered IP address to a registered IP address
from a pool (group) of registered addresses. The two subsets of dynamic NAT are
overloading and overlapping .

•

Overloading —Maps multiple unregistered IP addresses to a single registered IP address
by using different ports. Overloading is also known as Port Address Translation (PAT),
single address NAT, or port-level multiplexed NAT.

•

Overlapping —Maps internal IP addresses to outside IP addresses when the internal
addresses are possibly the same as the external addresses. Translates the internal IP
addresses to globally unique IP addresses. External addresses can also be mapped to
overlapping internal addresses.

Before configuring NAT, you must understand the following terminology:
•

Stub domain —Internal network that can use private IP addresses.

•

Public network —Outside the stub domain and resides in the Internet. Addresses in the
public network are reachable from the Internet.

•

Inside local address —Real IP address of the device that resides in the Internal network.
The stub domain uses this address.