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NAT (Network
Address
Translation)
Datagram
SINAUT MD740-1
In Network Address Translation (NAT) - often also referred to as IP
Masquerading - an entire network is "hidden" behind a single
device, the NAT router. This device is usually a router. The internal
computers in the local network remain hidden with their IP
addresses when they communicate to the outside via the NAT
router. For the external communication partners only the NAT
router with its own IP address appears.
However, in order for internal computers to be able to communicate
direct with external computers (on the Internet) the NAT router
must change the IP datagrams passing from internal computers to
the outside and from the outside to an internal computer.
If an IP datagram is sent from the internal network to the outside
the NAT router changes the datagram's IP and TCP headers. It
replaces the source IP address and the source port with its own
official IP address and its own, previously unused port. To this end
it creates a table showing the correlation between the original
values and the new ones.
When receiving a reply datagram the NAT router recognises by
means of the destination port specified that the datagram is actually
intended for an internal computer. Using the table the NAT box
exchanges the destination IP address and the destination port and
forwards the datagram to the internal network.
In the TCP/IP transfer protocol data are sent in the form of data
packets or datagrams. An IP datagram is structured as follows:
IP header
The IP header contains:
-
the IP address of the sender (source IP address)
-
the IP address of the recipient (destination IP address)
-
the protocol number of the protocol of the next highest
protocol layer (according to the OSI layer model)
-
the IP header checksum to check the integrity of the header
upon reception.
The TCP/UDP header contains the following information:
-
the port of the sender (source port)
-
the port of the recipient (destination port)
-
a checksum for the TCP header and some information from
the IP header (e.g. source and destination IP address)
TCP/UDP
Data
header
(payload)
Glossary
99 von 105
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