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Why are there three separate ranges of private IPv4 addresses of different sizes? Why not just reserve the largest (10.0.0.0 - 10.255.255.255) range and let people create /16 or /24 (or whatever) networks within that range if they need smaller networks or need to do subnetting? Is there anything wrong with having a huge number of available host addresses on your network?

19

Back when the RFC for private addressing was proposed, classful addressing was still common. The reasons for the three address ranges are found in RFC 1918, Address Allocation for Private Internets:

If a suitable subnetting scheme can be designed and is supported by the equipment concerned, it is advisable to use the 24-bit block (class A network) of private address space and make an addressing plan with a good growth path. If subnetting is a problem, the 16-bit block (class C networks), or the 20-bit block (class B networks) of private address space can be used.

  • So there's no drawback at all if a cautious admin wants to use IP addresses in the 10.0.0.0 - 10.255.255.255 range (with a suitable subnetting scheme in case they want to create subnetworks in the future) even on very small networks? – Adam Jun 9 '16 at 16:05
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    Correct. Modern business networking equipment defaults to classless routing. – Ron Maupin Jun 9 '16 at 16:11
3

While not an great answer (Ron Maupin's is more accurate), I was always taught to always use the the smallest class that left 10% room for expansion. This was "very important" with a facility that had many Subnets.

192.168.1.*
192.168.2.*
192.168.3.*

Could all be used to isolate entire security policies. For example the .1 network require the use of a proxy, .2 Maybe for management and bypass the proxy while .3 was the DMZ.

Some network equipment back then didn't even ask for a subnet. It just assumed you were using the entire class.

Eventually though that gave way to classless routing but it still is important to create segments that make easy sense.

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