ISRO CSE 2014 | Question: 6

Question

The process of modifying IP address information in IP packet headers while in transit across a traffic routing device is called

• A. Port address translation (PAT)
• B. Network address translation (NAT)
• C. Address mapping
• D. Port mapping

Answer 1

Answer : Network Address Translation

Network Address Translation (NAT) is designed for IP address conservation..Network address translation (NAT) is the process of modifying IP address information in IP packet headers while in transit across a traffic routing device. NAT operates on a router, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into legal addresses, before packets are forwarded to another network.

You can read more about it Here : 

http://www.cisco.com/c/en/us/support/docs/ip/network-address-translation-nat/26704-nat-faq-00.html

Comments

when a private ip address request reaches the router, then for the router to know , that what type of request is this,, (is it a apache web server request or to connect to another computer request).

hence we, that is private user with a private ip address has to enter the port number to let router know where to forward this request.

hence , converting this private address to router public address is called NAT

and specify the port to send it to a particular port tunnel is called port forwarding or port mapping

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what is the advantage of PAT over NAT ??

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NAT is just used to change private to public address .

PAT is an extension of NAT which not only changes but even assigns port to each address , which NAT cant do, hence we use PAT so that it can do whatever NAT does plus it assigns port address also ....

Answer 2

In IPv4, we have a shortage of addresses. There can only be $2^{32}$ different IP addresses possible, which was fine before, but now with increased reach of internet and each person having multiple devices, having $2^{32}$ unique addresses isn't enough.

NAT enables a user to have a large set of addresses internally, and one (or a few) addresses externally.

By internally and externally, we mean that a user inside a LAN or their own business space can have multiple IP addresses, which would be unique just to that network space.

These address sets are called Private addresses, and Internet has reserved it for users.

Private address sets:

• $10.0.0.0$ to $10.255.255.255.255$
• $172.16.0.0$ to $172.31.255.255$
• $192.168.0.0$ to $192.168.255.255$

Any user can have these IP addresses "internally" without the permission from any Internet authority.

 

When the packets from these users are sent to "the outside world"; in the IP packet the NAT router converts the private address (not globally unique) to a globally unique one.

Option B