NAT

[rohaanchaudhri]

Hi ! student here preparing for the CCST networking exam . ( First post )

NAT is also a great tool to use when an organization
changes its Internet service provider (ISP), but the networking manager needs to avoid the
hassle of changing the internal address scheme.

I read the text above in Todd lammle's book . the question i have here is why would you need to renumber internal IP address scheme in pre NAT systems since private and public IP address ranges are different and they do not overlap ?

[balaji.bandi]

we are not sure what document you reviewing there are many document and trainer all over.

the question i have here is why would you need to renumber internal IP address scheme in pre NAT systems since private and public IP address ranges are different and they do not overlap ?

High level we do NAT since RFC 1918 address not routable to Internet, so we do NAT using Public IP address.

You can learn more information as below :

https://www.cisco.com/c/en/us/products/routers/network-address-translation.html#~q-a

https://www.cisco.com/c/en/us/support/docs/ip/network-address-translation-nat/26704-nat-faq-00.html#:~:text=It%20enables%20private%20IP%20networks,are%20forwarded%20to%20another%20network.

suggest to post in learning community so can be address correctly :

https://learningnetwork.cisco.com/s/

[Joseph W. Doherty · Accepted solution]

Possiblity two things that remark has in mind, if you are using public ISP IPs, beyond one just for private to public, if you change ISPs all of them would need to change.  For example your company hosts its own public web and mail servers.  If such servers were also behind NAT, moving to a different ISPs would not require changing their host IPs.  (Unclear how this would be a huge advantage, as you would still need to deal with the change to their public NATted IPs.)

The second situation I'm unsure about.  Assuming all your IPs are public IPs and you have your own AS, there may be an issue if you move between global IP address block assignment regions.  I.e. what if you move from England to Australia, can you use your public IPs and AS?

[Flavio Miranda]

@rohaanchaudhri 

Sticking to your question. This situation happens, for example, when your company buy another company and they need to integrade their network, using some ISP service,  and for coincidence they use the same internal network range. On this case a NAT between them is necessary. 

 Antoher situation is when you hire a new ISP but they will not give you a public IP address. There might be different cases where you need NAT in order to avoid renumber internal networks.