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I had been working in a mid-large company, and had limited administrator privilege both server and network.

We had network that had two subnet on single shared network consist of 192.168.1.0/24 and 192.168.4.0/24. The full flagged administrator had been setup DHCP server with 2 pool 192.168.1.0/24 and 192.168.4.0/24.

I had been noticed that pool 192.168.4.0/24 not working even when 192.168.1.0/24 already full assigned.

Following picture of how our DHCP server look like. enter image description here

Are there way to solve this, so we can also use 192.168.4.0/24 when other exhausted ?

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  • It's not clear (to me) what you're trying to accomplish. Do you want two subnets, or just one?
    – Ron Trunk
    Commented Sep 15, 2015 at 12:25
  • your case as you described can't be valid . user can't assigned two subnets in the same broadcast domain (VLAN)
    – Gadeliow
    Commented Sep 15, 2015 at 13:21
  • @Gadeliow, why not? It is perfectly valid to apply secondary IP addresses to a router interface or SVI.
    – YLearn
    Commented Sep 15, 2015 at 13:32
  • i mean for client in certain VLAN not L3 interface
    – Gadeliow
    Commented Sep 15, 2015 at 13:35
  • Thank you all. Sorry yesterday I had trouble with our networking so I able to reply all of your answer today. @Gadeliow, well when I arrive on this company it already do so although our core switch manageable one but due to other switch unmanaged we not implement VLAN instead we implement two subnet in one broadcast domain.
    – dchochan
    Commented Sep 16, 2015 at 1:05

2 Answers 2

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There is quite a bit of detail missing from your description, but you do have a fairly simple question:

Can I implement dhcp relay on the router for two subnet in same broadcast domain?

You could, but there really is no point. DHCP relay features are only there to proxy specific broadcast traffic (by default only DHCP traffic) from one broadcast domain to another.

Since your client and your DHCP server are in the same broadcast domain, there is no need to proxy DHCP broadcasts. The server and client will be able to communicate with broadcasts just fine without any help.

A bit more commentary on other parts of your description, although we are still lacking details:

I as I know it would not be successful for subnet address that not used as IP address in router in this case 192.168.4.0/24 should not work.

There is no reason the router interface/SVI for the VLAN cannot have two addresses, one from each subnet. As such, it could work just fine. Even if this weren't the case, there are other reasons one may do something like this.

But since it not sunetted properly I had no idea what that full flagged administrator tried to achive

There is nothing wrong with the subnetting. There are two perfectly good subnets that do not overlap in your description. Although there may be other problems in the setup.

What he was trying to achieve we could only guess without knowing more detail and/or asking the party in question.

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  • Thank you for your answer @YLearn, currently the router do not have address for subnet 192.168.4.0/24 only had address 192.168.61.254 for our LAN there fore all device statistically using 192.168.4.0/24 subnet use gate way 192.168.1.254. I see the server log for DHCP the pool for 192.168.4.0/24 never used, is it cause by router do not have 192.168.4.0/24 subnet address?
    – dchochan
    Commented Sep 16, 2015 at 1:39
  • No. The DHCP server really doesn't know anything about the router. The reason that it isn't assigning addresses from that IP range would be because of how it is configured. Even if we knew which DHCP server you were using and had the configuration, that would be moving outside what is on topic on this site.
    – YLearn
    Commented Sep 16, 2015 at 2:01
  • look like I miss understand about broadcast domain concept. From the picture what I want to tell is there are two subnet on several unmanaged switch that connected by one router(in router it had been added in routing table it has 192.168.4.0/24 network under it). Since our subnet not separated by VLAN is there a way for our DHCP server to understand example PDA device want to use 192.168.4.0/24 as DHCP address.
    – dchochan
    Commented Sep 16, 2015 at 2:16
  • Yes that would be possible, however it all depends on how the DHCP server is configured. With ISC you would use multiple subnet declarations within a single shared-network declaration. With MS, they call them superscopes. Other DHCP servers may use different terminology. You would need additional configuration to specifically single out PDA devices. However, this again is getting way out of scope for this site.
    – YLearn
    Commented Sep 16, 2015 at 2:26
  • Thank you very much again for your answer I guess I will look for DHCP superscope configuration. It look like can solve the issue because I got the documentation from here technet.microsoft.com/en-us/library/dd759168.aspx
    – dchochan
    Commented Sep 16, 2015 at 2:39
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Thank you very much for your answer sir @Ylearn, the answer for my question can be solved by DHCP Superscope.

Following the picture what I really want try to achive. DHCP Superscope

Hope can be useful for other who had same problem :).

DHCP Superscope Explanation

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  • The only actual question you asked in your original post was "Can I implement dhcp relay on the router for two subnet in same broadcast domain?" That question is in no way answered by what you provide here, so perhaps you were asking the wrong question originally. If this provides the answer to your question, then you may want to edit your original post.
    – YLearn
    Commented Sep 16, 2015 at 4:21
  • Sorry it look like what I think and what I seek is different but my ultimate goal is to correct the configuration that seem wrong to me, and thank to you I got new knowledge about DHCP server :).
    – dchochan
    Commented Sep 16, 2015 at 6:08
  • @YLearn, I had been edited my original post once more thank you very much for your correction and answer.
    – dchochan
    Commented Sep 16, 2015 at 6:21

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