I have connected my smartphone to internet over 4G through Airtel (ISP)

I did - "What's my IP" on google. And below is the screenshot i obtained-

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I understand the public IP address is in the range 27.59.102.... because it needs to be seen and routable on the internet.

And the local address is also a public address (100.127.85....)!

Shouldn't local address be private address? (The approach used to overcome the depletion of IPv4 addresses)? As there will be millions of mobile phones and giving each of them an unique public IP address would deplete the address space very quickly. Isn't it?

Could someone help me in understanding what is happening in this scenario?

  • Questions about networks not under your control are off topic here. – Teun Vink Mar 26 '18 at 9:47

Your ISP uses Carrier Grade Nat (CGN) as defined in RFC 6598

As you can see in the RFC the network (which encompass your IP 100.127.85.X) is reserved for CGN, so while not a private network (as defined in RFC1918), it is used in the same way and is no longer a publicly routable network.

So you get a CGN IP address assigned to your device, and the ISP perform NAT with a globally routable address somewhere in its network.

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