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"The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets:

 10.0.0.0        -   10.255.255.255  (10/8 prefix)
 172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
 192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

I am really confused with the network prefix in private ip address. For example, Class C public ip address has network prefix of 24 bits, so a class public ip address can be represented as 192.0.0.0/24, so why private ip addresses use /16 prefix rather than /24 prefix?

also for class C private ip address, if the network bit is 16, how can it have 256 networks? shouldn't it just have one single network "192.168.x.x", because the first two octet for both starting address and ending address are "192.168" and take up all /16 prefix?

user25521292
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2 Answers2

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You are allowed to subnet or aggregate addresses in those Private address ranges however you would like. There is no such thing as address classes anymore. If you want one large 192.168.0.0/16 network, then that is fine. If you want to subnet that range into 512 /25 networks, you can, or subnet it into many different sized networks.

There is no longer the restriction to set networks that fall into the obsolete network address classes to specific network sizes. Subnet or aggregate them in any fashion that fits the rules of subnetting.

This two-part answer explains how to do IPv4 math.

Ron Maupin
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When they defined RFC1918, they reserved for private use 1 Class A network, 16 Class B networks, and 256 Class C networks. Even then, the era of classful networking was on the way out, but they codified the RFC recognizing legacy networks. With CIDR/VLSM, one can use those blocks anyway one wants. (eg. few people build a 10/8 LAN.)

Ricky
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