Today I presented about IP address during my online class based on Computer Networks by Andrew S. Tanenbaum and during QnA session, I was asked about private IP Address range. – – –

The questioner asked my why private IP address is determined in such range. I haven't really think much about the range because I accept it as something that had been decided by the people who came up with this technology but I eventually have to present again because I couldn't answer the questioner's question. Perhaps there are reason behind why the private IP address is determined in such ways?

For example why it starts with 10 and not 9? or why it starts with 172 and not 173?

I tried to read in RFC 1918 but I couldn't really find the answer about why they chose 10 or 172 or 192.

If anyone can share link that I could refer to or know the answer, I would highly appreciate it.

Thank you.

  • $\begingroup$ I don't think it'll be any more interesting than those were the next in line to be acquired in each class. $\endgroup$ Jul 14, 2020 at 13:54
  • $\begingroup$ The class A networks ran out pretty soon, some other parts were repurposed from unused ranges, exchanged for others more or less voluntarily with owners, or got back when the owners went under. $\endgroup$
    – vonbrand
    Jul 14, 2020 at 23:04

1 Answer 1


"The questioner asked my why private IP address is determined in such range"

IMO a good answer is "Because it has been established in RFC-1918; which is a "Best Current Practice" RFC :-)

The three ranges allow private networks to "resemble" the class A-B-C subdivision (and probably this is what the questioner wanted to hear from you). See older RFC791: .... To provide for flexibility in assigning address to networks and allow for the large number of small to intermediate sized networks the interpretation of the address field is coded to specify a small number of networks with a large number of host, a moderate number of networks with a moderate number of hosts, and a large number of networks with a small number of hosts..... Also see Wikipedia Classful network for more details / references.

The reasons for the particular numbers are not well known (and are only interesting from a historical-folklore point of view), and probably the main criteria was to use address spaces that were free/unused at the moment.

  • 10.x.x.x comes from the old ARPANET and is easy to remember
  • 172.x.x.x was marked by IANA as "Special purpouse" block of IP addresses
  • 192.168.x.x was used by Sun in its workstations (?!?)

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