CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

172.16.1.1
172.16.1.2
172.16.1.3
172.16.1.1, Port 1000
172.16.1.2, Port 2000
PAT
172.16.1.3, Port 3000 150.1.1.1, Port 1002
NAT Table of PAT router
Inside Local Inside Global
172.16.1.1:1000
172.16.1.2:2000
172.16.1.3:3000
Figure 17-6: Port Address Translation
- Overlapping Address Translation is a special NAT operation that is being used when
addresses in the inside network overlap with addresses in the outside network, which can exist
when an organization does not use private addressing (illustrated in Figure 17-7), or when 2
companies that have similar IP addressing scheme merge due to business requirements.
200.1.1.2
Private
150.1.1.1:1000
150.1.1.1:1001
150.1.1.1:1002
192.168.1.1
Figure 17-7: Overlapping Address Translation
- Figure 17-7 shows a network with overlapping addresses. Whenever the client sends a packet
destined to www.yahoo.com (200.1.1.1), the packet would never arrive at the server, because the
client might assume 200.1.1.1 is on the same LAN with it, hence it would not even try to
forward packets destined to 200.1.1.1 to its default gateway!
- NAT is able to solve this problem by translating both the source and destination addresses of
the packet passes through the NAT router, by having a NAT static mapping configuration for
translation between the outside global address – 200.1.1.1, and the outside local address –
192.168.1.1, which represents the real outside host to the inside network.
- The client first sends out a DNS request to www.yahoo.com. When the DNS reply comes back
from the outside network, the NAT router would intercept and change the DNS reply so that the
client in the private network would think that 192.168.1.1 is the IP address of www.yahoo.com.
128
150.1.1.1, Port 1000
150.1.1.1, Port 1001
NAT
Internet
150.1.1.1
200.1.1.1, Port 80
200.1.1.1, Port 80
200.1.1.1, Port 80
www.yahoo.com
200.1.1.1
Source Destination Source Destination
200.1.1.2 192.168.1.1 data 150.1.1.1 200.1.1.1 data
Source Destination
Source Destination
192.168.1.1 200.1.1.2 data 200.1.1.1 150.1.1.1
data
200.1.1.1
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com