How to reserve IP addresses for wireless devices

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.

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.

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.

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,

pritamdutt: told very good.

You have only one option.

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.

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

Janibek, just buy a cheap linksys router like the RV082 and sepearate the segments, very easy.

@Janibek, can you please share the Make and Model number of your AP.

it's a DLink DAP-1522, it's an AP mainly, but is capable of being a DHCP server as well.

Janibek,  you could also use a layer 3 switch if you have one  to create the wirless subnet.

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,

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!

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

Alright, mlchelp, what's your solution?

@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!