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How to reserve IP addresses for wireless devices

Posted on 2011-10-16
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Last Modified: 2013-12-09
Hi experts,

We have a DHCP server running on Windows 2003 standard. Let's say we have a scope with the range from 192.168.0.10 to 192.168.0.170
How do I reserve addresses between 192.168.0.150--170 to be assigned only for the wi-fi devices(phones, laptops, tablets etc.)?

I believe If I just enable DHCP on the router, and set the scope on it to the desired range that might lead to both Windows DHCP and router DHCP assigning the same address, correct?

Please let me know your vision of the solution? Maybe a separate scope might be a better solution?

Many thanks!
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Question by:Janibek
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pritamdutt earned 600 total points
ID: 36977702
Ok there are two ways to approach your requirements.

1. You could do MAC binding of Wireless Devices (if you know and wish to control who actually connects to your network)
2. You could the required Wireless IP Addresses in the Exclusion List of Scope: To Add IPs in exclusion list Expand the Scope -> Right Click on the Address Pool and Click on New Exclusion Range...Once the IP Addresses are added to exclusion range on DHCP Server, you can safely use your Wireless Router to allocate these IPs to wireless devices.


Hope this helps.

Regards,
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Expert Comment

by:mlchelp
ID: 36977706
You can only have one dhcp server on each subnet, you could create two subnets, one for wireless and one for wired, and that would work. You could set up two scopes but I’m not sure how you would be able to determine whether the dhcp request was coming from the wireless devices. Why do you care which device gets which address, if its so you can tell by ip address if the device is wireless or wired then you’re going to have to create a new subnet to use exclusively by the wireless devices.
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by:mlchelp
ID: 36977713
pritamdutt:

What is to stop the AP from handing out a address to a wired client or the dhcp server handing out the address to a wireless client, this will not work, it has to be two different subnets IE broadcast domains, either unit can answer the dhcp broadcast and give an address.
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Expert Comment

by:mlchelp
ID: 36977737
You can have 2 dhcp servers as long as the scopes do not overlap but there is no way to determine which server will answer first so you don’t know which ip the clients will get. if there is some way to block the dhcp request from entering the wired lan and block the wired lan dhcp request from getting to the Ethernet port on the AP's then this would work but I can’t see any way you would be able to accomplish this.
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by:pritamdutt
ID: 36977751
Hi @mlchelp

Please note
1. When specifying DHCP Server, you specify the appropriate interface and range. In this case, while enabling DHCP range if you specify the Wireless interface, there is no way for AP to handout IP Address from defined IP Address Range to a wired client.  

2. Yes its best to define different subnets for segregation of clients, but then still the question would be
- Where would be the DHCP Server for wireless clients on AP or Windows 2003 Server? If you say AP then your current comments could still apply. If you say Windows 2003, then question is @Janibek's AP & network capable of handling VLANs to segregate the subnets?

If you seek the best way to design a network I would recommend the following:

1. Have separate Subnets Segregated by VLAN's
2. Should use common DHCP Server such as Windows 2003 Server
3. Should have effective mechanisms in place to prevent unauthorized access to wireless network.

Hope this clarifies.

Regards,
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Expert Comment

by:mvsnaniou
ID: 36977764
pritamdutt: told very good.

You have only one option.
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by:mlchelp
ID: 36977778
pritamdutt:

Please note

 1. I’m not aware of any AP's that have a dhcp server in which you can pick a certain interface, please feel free to enlighten me with the model number. Still what would prevent the 2003 server from answering the dhcp request?

2. If he made two subnets, then the WAP would hand out dhcp to the wireless segment and the regular dhcp server would hand out addresses to the wired clients on the other subnet, the key is that the router in between the subnets will not pass the dhcp request unless configured to do so.
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Author Comment

by:Janibek
ID: 36977779
mlchelp, nice "gotcha" on the DHCP requests over the wired LAN!

pritamdutt, I was gonna stick to the second option, because the first one isn't feasbile due to large amount of wi-fi devices. How do I avoid the "collisions" between DHCP servers when a request comes over the wired LAN, and both wi-fi router's DHCP and windows DHCP tries to assign an IP address? I know one would be ignored due to an ACK  from the requester, but I need to guarantee that only wi-fi devices get IPs reserved for the wi-fi router, and the wired ones get IP from the regular DHCP.

thanks
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by:mlchelp
ID: 36977789
Janibek, just buy a cheap linksys router like the RV082 and sepearate the segments, very easy.
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by:pritamdutt
ID: 36977793
@Janibek, can you please share the Make and Model number of your AP.

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Author Comment

by:Janibek
ID: 36977813
it's a DLink DAP-1522, it's an AP mainly, but is capable of being a DHCP server as well.
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by:mlchelp
ID: 36977820
Janibek,  you could also use a layer 3 switch if you have one  to create the wirless subnet.
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by:pritamdutt
ID: 36977846
Hi,

I have checked the Installation guides of DAP-1522 and DAP-1522_revB and have learnt that  only DAP-1522_revB AP has the capability to perform as DHCP Server.

Just for curiosity sake, is it possible to enable DHCP on your AP, and connecting it to isolated PC to test if the machine on wired  lan is recieving IP Address from AP?

I am just curious, as I have never encountered this issue, may for the reason that I have always believed in best designed networks!

Regards,

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Author Comment

by:Janibek
ID: 36977911
Alright, pritamdutt, I enabled DHCP on wi-fi router, specified the range of assignable IPs and enabled  proper reservations on the regular Windows DHCP server.

I've tested DHCP over the wired LAN and it seems that everything works fine -- IPs are assigned from regular DHCP, and wi-fi connections get IPs from router's DHCP.

Let's see if it works cool for a few days(IP lease is set to 24 hours on DHCPs) and if so i'll close the thread and set the scores. Thanks for you input guys!
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by:mlchelp
ID: 36978977
Janibek, It won’t work that way for long, like I said there is no way for this to work, eventually you will have an issue,. Even in pritamdutt's phantom router case that can somehow miraculously restrict the dhcp scope to the wireless interface, there would still be no way to keep the dhcp  packets from the lan from entering wifi. This is basic networking.

II have seen many people make this mistake, It’s like the guy at home that brings there linksys wireless router to work, and plugs it into the network in his office because he thinks it would be cool to have wireless. Then it starts handing out a different ip range to Pc's on his lan and causes a large problem LMAO
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by:Janibek
ID: 36979208
Alright, mlchelp, what's your solution?
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by:mlchelp
mlchelp earned 400 total points
ID: 36981010
you need to seperate the wireless from the wired network with a router or a layer 3 switch. one side of the router will be 192.168.0/24 and the other side or wireless side will be 192.168.1/24 then you can have dhcp servers on both networks.
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Author Closing Comment

by:Janibek
ID: 37061373
@pritamdutt, your solution is easier to implement in my case and was actually implemented.

@mlchelp, your solution is better, but harder to implement in my case, because I only have layer 2 switches.

Guys, thanks to both of you for your efforts. Really appreciated!
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