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I'm working on a private network which uses static IPv4 addresses as well as DHCP addressing for the physical LAN network.

At a previous company I worked at they would assign static addresses in the 10.*.*.* space and all DHCP/LAN addresses were assigned in the 192.168.*.* space. Both of these address spaces are defined in the IPv4 private address space and there were never any internal conflicts.

From personal experience at home, school, at work, and pretty much any other machine I've dealt with extensively (Windows and a few Linux distros), the DHCP server would always by default choose an address from the 192.168.*.* address space.

Now my question is can I rely on this behavior? Do DHCP servers always by default assign from the 192.168.*.* pool (or any pool other than the 10.*.*.* pool), leaving the 10.*.*.* pool free for private static addressing? If not, under what conditions might a DHCP server choose an address in the 10.*.*.* address space?

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2 Answers 2

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I agree that it's very common for DHCP servers (especially in consumer-grade equipment) to ship ready to hand out DHCP leases from the 192.168.0.0/16 network, but there's no RFC nor any requirement that they should do so.

I think you would be very unwise to assume that no piece of equipment will ever hand out leases from 10.0.0.0/8. Moreover, networks with multiple random DHCP servers are the bane of many a network admin's existence; if you think there might be an issue here, the very first sensible step is to hunt down and turn off any DHCP servers you don't control, and make sure those you do are well-configured and operate in accordance with your network design.

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There is no reason to select 192.168/24 over 10/8 or 172.16/12. No non-SOHO DHCP server I worked with has defaulted to 192.168 yet.

Home stuff on the other hand seems to think that 192.168/24 is the only thing in existence.

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The RFC1918 network that starts 172. is in fact 172.16.0.0/12, not /20. But apart from that I agree with you (or, to be precise, had already said much the same thing!). –  MadHatter Nov 2 '12 at 9:43
    
Will fix. (Note to self, check values you do not often use.) –  Hennes Nov 2 '12 at 9:44
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That's why I love 172.16/12 and use it often; no-one else uses it! Using it for a SOHO network makes VPNs to client sites much easier, as there's hardly ever an address overlap. –  MadHatter Nov 2 '12 at 9:45
    
I agree, it prevents future problems. –  Hennes Nov 2 '12 at 9:47
    
I suspected as much. I didn't find any literature which required it to be so, but it did seem odd that a major business and university for the most part selected from 192.168. Perhaps dumb luck or behind-the-scenes management to ensure this behavior :P –  helloworld922 Nov 2 '12 at 9:48

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