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There are numerous reference about private IP address range which can not be routed to the internet. They are:

In class-A: 10.0.0.0 to 10.255.255.255 (10.0.0.0/8) In class-B: 172.16.0.0 to 172.31.255.255 (172.16.0.0/12) In class-C: 192.168.0.0 to 192.168.255.255 (192.168.0.0/16)

These are said to be used for devices within the private network.

I would like to confirm my understanding that among the publicly available IPV4 address space, these private address ranges are still available.

For example, if we take this address 172.31.252.252, this comes under the private IP address range of Class-B.

Suppose if I build my own private network, I can use this for a device in my network.

But this address (172.31.252.252) is still available in the global IPV4 address space. Am I correct? If this is not correct, is there any reason why this address has been taken off from global IPV4 address space? (I mean the whole range of private addresses mentioned above)

Thanks!

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    As you said in your first sentence, private addresses [are not] routed over the public Internet. So you can do with them as you wish. Also, please remember that address classes are obsolete, and have been since before you were born. Please let them die in peace.
    – Ron Trunk
    Dec 13, 2023 at 16:14
  • Thanks Ron, for patiently answering my question. May I know why there are reservations on private IP ranges please? why can't we use all the IP address in a private network? So if the classful addressing is obselete, then how routers in a network understand the network part from the IP address? In classfull address, the frst four bits in the first octat help to identiy the n/w portion. In CIDR, we refer the n/w portion with a slash at the end (x.y.z.w/k). But this notation is for our own understanding. Routers dont get it. am I correct? Dec 13, 2023 at 16:29
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    First: If your network is completely private, that is, not connected to the Internet, then you can do whatever you want. But if you want to connect to the Internet, you have to follow the Internet rules (RFCs). And RFC 1918 says that specific ranges are reserved for private use and won't be advertised over the Internet. Note that this is by agreement -- routers don't know the difference.
    – Ron Trunk
    Dec 13, 2023 at 16:45
  • Second: Router route tables have a network and a (variable) subnet mask. That's how they determine how to forward the packet. IP packets do not have any subnet information in them.
    – Ron Trunk
    Dec 13, 2023 at 16:49
  • Ah, ok. So when we configure a router for a network, we need tell to the router that in an IP address x number of bits are for network and 32-x number of bits for host IP address. am I correct? By the way, I am just trying to visualise the global internet and different private networks we have in home, office etc and trying to get an understanding of how they communicate. Dec 13, 2023 at 17:09

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