Network Address Translation which convert the Private IP address to the Global IP address.

1. C.K.PITHAWALLA COLLEGE OF ENGINEERING
AND TECHNOLOGY, SURAT.
(5th sem. Computer)
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2. NETWORK ADDRESS TRANSLATION (NAT)
Providing IP level access between host at a site and the rest of the Internet,
using valid globally valid IP address known as NAT.
The address used for private network are strongly recommends the
following address be used:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
These address never appear on the internet.
Network address translation (NAT) allows a site to use a set of private
addresses for internal communication and a set of global Internet
addresses for communication with another site.
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3. The site must have only one single connection to the global Internet through a
router that runs NAT software.
Packet before Packet after
translation translation
1
W
e
2 NAT
b
10.0.0.1 198.1.1.2
S
LAN e
r
v
3 e
This software is known as
r
NAT box, all datagram
4 pass through the NAT box
as they travel form the
site out to the Internet or
form Internet to site.
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4. What the NAT do?
NAT translate the address outgoing datagram replacing the each source
address by to the Global Address.
1
NAT
W
e
2 b
10.0.0.1 198.1.1.2
S
e
r
v
3 e
Source Address Global Address
r
4
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5. What the NAT do?
continue…
NAT translate the address incoming datagram replacing the each Global
address with the private Address of the correct host.
1
NAT
W
e
2 b
10.0.0.1 198.1.1.2
S
e
r
v
3 e
Private Address Global Address
r
4
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6. NAT Translation Table Creation
How the NAT know which Internal host should receive a datagram that
arrives form the Internet. So the NAT maintain a Translation Table that is
used for mapping.
Each Entry of the Table specifies the two item:
1. IP address of a host on the Internet.
2. The Internet IP address of a host at the site.
There are several way to initialize the table:
1. Manual Initialization.
2. Outgoing datagram's.
3. Incoming name lookups.
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7. Continue…
1. Manually Initialization: Configures the translation table manually
before any communication occurs.
2. Outgoing datagram: When it receive a datagram for the internal
host, NAT create an entry in the translation table as a record of the
both host and the destination addressees.
3. Incoming name lookup: It is built as a side-effect of handing
domain name lookups. When a host on the Internet looks up the
domain name of an internal host to find its IP address, the domain
name software creates an entry in the NAT translation table, and
then answers the request by sending address G. Thus, from outside
the site, it appears that all host names at the site map to address G.
Out of this three the mostly implementations of NAT use outgoing datagram
to initialize the table.
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9. Multi Address NAT
As we have simplistic the NAT perform a 1-to-1 address mapping between
external address and internal address.
But NAT want to permits concurrency by retaining the 1-to-1 mapping, by
allowing the NAT box to hold multiple Internet address.
This is Known as Multi-Address NAT which is done by the NAT box. It set
of K globally valid address G1,G2,…,Gk.
Thus, multi-address NAT allow up to K internal host to access a given
destination concurrently.
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10. Port Mapping NAT
NAT provide concurrency by translating TCP or UDP protocol port
number as well as address.
It is known as Network Address Port Translation (NAPT), Which is
included in the translation table With the source and destination IP
address.
Private Private External External NAT Protocol
Address Port Address Port Port Used
10.0.0.5 21023 128.10.19.20 80 14003 TCP
10.0.0.1 386 128.10.19.20 80 14010 TCP
10.0.2.6 26600 207.200.77.200 21 14012 TCP
10.0.0.3 1247 128.210.1.5 80 14007 TCP
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11. Continue…
The table contain the four internal computers that are currently accessing
destination on global Internet and all of them are using TCP.
In the table two internal host accessing protocol port 80 (web server) on
computer 128.10.19.20.
However the source port cannot be guaranteed it turn the two internal
host have same port number. To avoid this NAT assign a unique port
number to each communication that is used on the Internet.
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12. Continue…
As in the table the first two item correspond to the TCP connection.
Such as:
10.0.0.5, 21023, 128.10.20, 80
10.0.0.1, 386, 128.10.20, 80
After the computer in the Internet that receive datagram after the NAPT
perform the translation such as:
G, 14003, 128.10.20, 80
G, 14010, 128.10.20, 80
Where G is the Globally valid address of the NAT box.
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13. Pros and Cons Of the NAT
Pros: Cons:
NAT increases the design IP addresses and ports lose
space available for an their end-to-end significance.
intranet.
Increases amount of state
Saves address space. information in the network.
Can be deployed
incrementally and (almost) Increases the complexity of
transparently. an edge router (protocol
dependent).
Method of choice for home
network. Some protocols don‘t work
across NAT’s.
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14. Conclusion
NAT can be static or dynamic.
Uses a set of predefined private addresses.
Conserves legal IPv4 addresses.
NAT plus PAT often used .
PAT uses unique source port numbers on the inside global IP address to
distinguish between translations.
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