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NAT, PAT and NAPT
12
NAPT
Network Address & Port Translation (NAPT) is the most popular form of address
translation. It is used almost exclusively by access devices designed to hide small-to-
medium sized networks behind a single public IP address. NAPT works by translating
the source IP address and the source port number on the public interface.
`
192.168.0.1
HTTP 30.0.0.1
Src : 192.168.0.1/600
Dst: 30.0.0.1/80
Src : 30.0.0.1/80
Dst: 192.168.0.1/600
In case a HTTP session is initiated which has to pass the NAPT enabled box, both the
source IP address and source port number will be translated for outgoing packets.
For incoming connections (belonging to the outgoing connections), the destination IP
address and port number will be changed.
`
192.168.0.1
`
192.168.0.2
Figure 4:
Suppose that two hosts want to share one common IP address. For outgoing traffic
there will be no issue: both the IP addresses "192.168.0.1" and "192.168.0.2" are
translated into this same IP address. But, as soon as packets come back (incoming),
the NAPT box has to know to which of the two 192.168.0.x addresses the address
translation needs to be performed.
This is where port translation comes into action: the destination port number in the
incoming packet will be used as input to decide to which of the 192.168.0.x
addresses to translate the address.
Private
Public
192.168.0.1/600
20.0.0.11/1025
TM
SpeedTouch
192.168.0.254
20.0.0.1
NAPT enabled
Src : 20.0.0.1/1025
Dst: 30.0.0.1/80
Src : 30.0.0.1/80
Dst: 20.0.0.1/1025
Figure 3:
NAPT example
Private
Public
192.168.0.1/734
20.0.0.1/403
192.168.0.2/521
20.0.0.1/908
TM
SpeedTouch
192.168.0.254
20.0.0.1
NAPT enabled
NAPT for multiple hosts (share the same IP address)
Public
30.0.0.1
Public
30.0.0.1
E-NIT-CTC-20040716-0004 v1.0
`
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