I just read that specific ranges of IPs have been set aside to be used to private ip addressing. But if we already distinguish between public and private IPs why do we need to reserve space? Why cant a public ip and a private ip be the same considering other publics ips do not access the private one directly?


1 Answer 1


You actually could use public address space on your private network (nobody is preventing you from doing this), but that would be foolish since you could no longer reach those addresses on the public Internet. Any attempt to reach public Internet sites with addresses in the range you use for your private addresses would never leave your private network.

The reason for the separate private address space is that it will not conflict with any public IP addresses.

  • Ah so we can still determine if we were trying to communicate with a device on the local network or foreign device by reserving this space. Correct? Dec 19, 2015 at 2:36
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    IP was originally designed so that every host would have a unique address. Unfortunately, the number of IPv4 addresses wouldn't come even close to supporting that today. The RFC 1918 private addresses were set up so that we could have unique private addresses within a network, and still have unique public IP addresses on the Internet. This requires NAT, which is a kludge. All this was to stall for time until the next version of IP (IPv6) becomes prevalent. IPv6 allows an almost unlimited number of IP addresses, and it fixes the address shortage.
    – Ron Maupin
    Dec 19, 2015 at 2:40
  • @RonMaupin: Once upon a time someone thought 32-bits of address space was enough. Some day the effective size of IPv6 address space will also be a problem (as the effective size is a lot less than the total address space, due to minimum allocation size limitations). Definitely not unlimited. Dec 22, 2015 at 4:14
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    @NickBastin, as Vint Cerf, father of IP and the Internet told me, we will be using something else long before we need to worry about IPv6 address exhaustion. If there are 17 billion people in 2100, there will be over 2000 /48 networks per person, with 65,536 subnets per /48, and each subnet can have over 18 quintillion addresses. That assumes we only use the 1/8 of the IPv6 address space currently assigned for global addresses.
    – Ron Maupin
    Dec 22, 2015 at 4:27

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