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I've been reading about IPv4, networking, subnetting and the different classes of addresses.

I understand that IP addresses beginning with numbers from 1 to 126 are considered "Class A" addresses.

However suppose I created a private network for my company (accessible to the internet through a router/gateway) of 100 people, is there anything stopping me from giving out addresses like 15.0.0.1 - 15.0.0.100 ?

Would this be a bad idea for some reason, or does it not matter at all?

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    Network classes are dead, killed in 1993 by RFCs 1517, 1518, and 1519, which defined CIDR (Classless Inter-Domain Routing). Modern networking doesn't use network classes. Please let them rest in peace. – Ron Maupin Feb 13 '18 at 23:54
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    @RonMaupin: While you are absolutely correct, we still get new employees fresh out of education who have just been taught classful networking like it was invented yesterday. It's remarkable. – Lightness Races in Orbit Feb 14 '18 at 2:43
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    @CodyBugstein: That is why we do not learn professional engineering skills from random tutorials on the internet – Lightness Races in Orbit Feb 14 '18 at 2:43
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    @CodyBugstein: Even if he has 20 years of experience, that still means he started teaching in 1998. Which is 5 years since the end of network classes. – MSalters Feb 14 '18 at 8:57
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    @CodyBugstein: He may be a professional teacher of 20+ years. He's still teaching stuff that became irrelevant years ago. You're right, maybe somebody needs to tell him :-) – psmears Feb 14 '18 at 9:20
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As long as you are translating your "15.0.0.0" address space to something unique on the Internet that doesn't overlap, things will "work fine".

However, you won't be able to communicate (easily) to any users who own "the real" 15.0.0.0/8. At the moment some of that space seems to be owned by HP:

$ whois 15.0.0.1
[Querying whois.arin.net]
[whois.arin.net]

NetRange:       15.0.0.0 - 15.103.255.255
CIDR:           15.96.0.0/13, 15.64.0.0/11, 15.0.0.0/10
NetName:        HP-INTERNET
Organization:   Hewlett-Packard Company (HP)

That is why this approach is not recommended. Your users (who don't know any better) will, for reasons they won't understand, not be able to speak to random companies on the Internet -- like HP in the 15.0.0.0/8 example above.

As Ron Maupin pointed out, the recommended addresses to use on the internal network are the RFC 1918 addresses. Since no one is permitted to use those addresses on the Internet, there will never be another entity you won't be able to communicate with.

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    Hewlett-Packard used to own all of 15/8 and 16/8. Since 2015 the situation is probably more complicated. – OrangeDog Feb 14 '18 at 14:54
  • @OrangeDog Yup. The whois above only shows a /10, /11, and /13 in use by HP. I didn't whois the rest remaining blogs because I assumed the OP threw 15.0.0.0 - .100 out there as an example, and not as the specific one they were intending to use. – Eddie Feb 14 '18 at 18:41
  • ^remaining *blocks, rather – Eddie Feb 15 '18 at 16:55
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Any network addresses you use in your own company that are in use or assigned to a different company on the public Internet will be inaccessible to your users trying to reach those addresses on the public Internet, and you will not be able to advertise to the public Internet any addresses assigned to a different company.

There are three ranges of private addresses assigned by RFC 1918:

  1. 10.0.0.0/8
  2. 172.16.0.0/12
  3. 192.168.0.0/16

You are free to use those within you own company if you NAT the addresses when using the public Internet.

To do business on the public Internet, you need to be assigned your own unique public address(es).

  • Right, but so long as the addresses I use are behind a router with a unique IP on the internet, I can use whatever addresses on my machines as I want right? – CodyBugstein Feb 14 '18 at 2:04
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    As long as you don't EVER need to talk to the Real Internet Hosts(tm) with those addresses, sure. – Ricky Beam Feb 14 '18 at 2:09
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    If the private address ranges are too small for your network, I'd love to see your layout. Combined, that's 2^24+2^20+2^16 = almost 18,000,000 addresses! – ErikF Feb 14 '18 at 6:13
  • If those ranges are too small for your network you probably have enough infrastructure to set up private NAT on routers within your network, instead of having to use addresses outside those ranges. – IllusiveBrian Feb 14 '18 at 16:13

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