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I recently learned about private networks. From what I understand there are three address ranges for use in private networks:

  • 10.0.0.0/8
  • 172.16.0.0/12
  • 192.168.0.0/16

If this is the case, then why on earth is pretty much every SOHO router configured to use either 192.168.1.0/24, or in some cases, 192.168.0.0/24?

My question is:

  • Doesn't this mean the configuration of most SOHO routers are wrong? Does this have any serious consequence? Should we reconfigure our routers to use standard CIDR blocks?

  • If we want to be compliant, the smallest private CIDR block 192.168.0.0/16 can accommodate 65,534 hosts (I hope my math is right.) This seems much too large for most SOHO networks. What should we do in this case?

  • 2
    What do we care n a site about professional administration about end user SOHO low quality stuff? – TomTom Mar 20 '14 at 15:50
  • They are choosing an arbitrary class C /24 subnet. What you posted in your OP is the entire range of 192.168 addresses allowed for private use. – TheCleaner Mar 20 '14 at 15:54
  • Answers (in order of your "question marks"): 1. Nope 2. Nope 3. Do whatever you want within the confines of the router's allowed config and RFCs 4. Wrong. CIDR is "classless", hence the smallest private CIDR block in the 192.168.0.0 network is any /32 if you want to get picky, or any /30 if you want an actual 2 way communication. – TheCleaner Mar 20 '14 at 16:00
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From RFC 1918;

the first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B network numbers, and third block is a set of 256 contiguous class C network numbers.

Thus, 192.168/16 assumes it will be subnetted into a /24 such as 192.168.1.0/24. This network would have a maximum of 254 hosts. (256 minus broadcast and network address)

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  • Ah, I see my mistake! Thanks very much for clearing that up for me. – AlfaZulu Mar 20 '14 at 16:07

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