IP Addressing – Using Class A Addresses on a Private Network

ipip addressipv4nat;subnet

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?

Best Answer

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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