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I had a smart (but non-technical) user ask me today:

Why did they pick 192.168.. for a private network address?

The only answer I could come up with is because 192 = 11000000 in binary. And 168 is 10101000 in binary. Both of which are kind of cool looking.

Is there a real historical reason for that particular choice of numbers? Why not 127.127..? Or 128.128..?

Similar question for 10.0.0.0 and 172.16.0.0

Thanks!

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the way you look at it, all binaries will look good :) 101000 1000100 1000010 – Devrim Sep 10 '09 at 15:03
1  

Before classless networks were invented, it was decided to make three ranges of private addressing space. These were:

  • Class A: 10/8 (the old ARPA reservation)
  • Class B: 172.16/12 (one of the first available class Bs)
  • Class C: 192.168/16 (one of the first available class Cs)

There are:

  • 1 Class A private prefixes (16.7 million addresses)
  • 16 Class B privates (65536 addresses each, totalling ~1 million addresses)
  • 256 Class C privates (256 addresses each, totalling 65536 addresses)

It is important to note that "Classes" haven't existed since 1994, and these days we use CIDR, which has a variable length subnet mask.

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how does this answer the question how they chose "192.168"? – Philipp Keller Mar 8 at 21:26

It appears they where selected by IANA simply because they where unused. The earliest mention I can find in an RFC is 1597. Also see rfc 1627. Both rfc have been obsoleted by rfc 1918

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I forwarded the question to the internet-history mailing list and Craig Partridge, chief scientist at BBN, said:

10.0.0.0 is easy. For folks who needed LARGE private networks the only large space available by the early 1990s was the old ARPANET network number (the ARPANET was net 10 and was decommissioned around 1991).

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127.0.0.0/8 is a lookback subnet and thus cannot be used as a network spread around multiple machines. Similar applies to 128.x.x.x and other subnets which afaik are predefined by IANA to fall into the a, b, or c class of subnets based on their prefixes. This is probably the cause why 10.x.x.x, 192.168.x.x, etc are so wide spread - because they fall into different subnet classes. The numbers itself then were not chosen for a specific reason, they were simply unassigned or previously reserved as Zoredache already answered.

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The answer is actually alluded to in RFC 1918 (which established the 10.0, 172.16 and 192.168 ranges)

When the internet was invented, it's creators were really focused on computers connecting to each other across the public internet. The idea of "private internets" wasn't really factored into the addressing system.

When enterprises began using private networks, a real problem arose: The IANA was running out of IP addresses. Private networks that needed to communicate with the public internet were in a bind... if they registered an IP Address for their private network, then this reduced the pool of IP addresses that others could use, but if they didn't, then there was no guarantee that the IP Addresses they ended up using for their private network wouldn't conflict with some external service that the company would eventually want to use.

To solve this problem, the IANA reserved an unused block of IP Addresses for private networks. Enterprises (and, eventually, home networks) could use these addresses with the certainty that they'd never conflict with a public internet IP Address.

Here's the relevant text from the RFC:

1. Introduction

For the purposes of this document, an enterprise is an entity autonomously operating a network using TCP/IP and in particular determining the addressing plan and address assignments within that network.

This document describes address allocation for private internets. The allocation permits full network layer connectivity among all hosts inside an enterprise as well as among all public hosts of different enterprises. The cost of using private internet address space is the potentially costly effort to renumber hosts and networks between public and private.

2. Motivation

With the proliferation of TCP/IP technology worldwide, including outside the Internet itself, an increasing number of non-connected enterprises use this technology and its addressing capabilities for sole intra-enterprise communications, without any intention to ever directly connect to other enterprises or the Internet itself.

...

3. Private Address Space

...

An enterprise that decides to use IP addresses out of the address space defined in this document can do so without any coordination with IANA or an Internet registry. The address space can thus be used by many enterprises. Addresses within this private address space will only be unique within the enterprise, or the set of enterprises which choose to cooperate over this space so they may communicate with each other in their own private internet.

As before, any enterprise that needs globally unique address space is required to obtain such addresses from an Internet registry. An enterprise that requests IP addresses for its external connectivity will never be assigned addresses from the blocks defined above.

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