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I came across this network on a public WiFi on a Railway Station. I wanna know the network and the details and most specifically, how I was able to get an IP from the range 100.x.x.x as Private IP ranges are 10.x.x.x, 172.16.x.x-172.31.x.x & 192.168.x.x with subnets of 8, 16 and 24 respectively. I know there is a use of CIDR networking in this case due to which I can find out the number of networks and hosts using operation on individual bits. But how am I able to get an IP from a public IP range is what I'm intertested in knowing. A complete breakdown of the whole network will be highly appreciated. Thanks.

Stuggi
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Saki Osive
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1 Answers1

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IANA assigned the 100.64.0.0/10 IP address block to ensure the proper working of carrier-grade NAT (CGN). Many companies often use this space incorrectly as a private IP space, instead of the ranges specified in RFC1918.

RFC6518 address space is designed to be used between a carrier router's downstream interface to their customer access network and the customer's router on the WAN interface, in order to allow the carrier to conserve public IP space. They would then use NAT on the carrier router, hence the term "carrier-grade NAT".

IP Space Example

It is possible to use any IP address prefix on a DHCP scope, as DHCP does not distinguish what is and what isn't a publicly routable prefix. Many companies (including the likes of Microsoft Azure and Amazon Web Services) utilise DHCP for the distribution of publicly routable IP addresses to virtual machines on their networks as part of their service offerings.

It is strongly not recommended to use publically routable IP address blocks in private networks, due to the exhaustion of resources globally, and most Regional Internet Registries (RIRs such as APNIC, ARIN, RIPE NCC, etc.) will actually revoke licenses to use public IP space if a use case is not suitable or sufficient.

And just to help you better your understanding of RFC1918 Private IP Space, the three prefixes are:

  • 10.0.0.0/8 (10.0.0.0 to 10.255.255.255)
  • 172.16.0.0/12 (172.16.0.0 to 172.31.255.255)
  • 192.168.0.0/16 (192.168.0.0 to 192.168.255.255)

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