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RFC 1918 specifies 3 ranges of IP addresses as private

10.0.0.0/8

172.16.0.0/12

192.168.0.0/16

So in a real network, only these IP addresses could be used? What if I assigned a host on the network an IP of 12.1.1.1, for example. Would there be a problem?

2 Answers 2

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You could, but you shouldn't. If you do so, the real devices and services using that IP range become unreachable.

I've seen a number of customers make this mistake (sometimes intentional, sometimes not), and it always ends up with renumbering the internal network because the public IP's used internally needed to be accessed.

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Indeed you can but you shouldn't. What happened in the past, at the beginning of the Internet, was that companies rented public C classes from IANA because it was cheap ('90s/2000's)and used that to build internal network; then they stopped paying for such public IP addresses but didn't spend time to renumbering it; the result was what @teun said, i.e. that it was not possible for them to reach the public IPs used internally. And yes renumbering a network is a pain in the neck...

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