Wireless Access Points: DHCP Operation

I have come to realize that not all access points are created equal.  So, I'd like to parse the space into sets of capabilities and, with limited experience, could use inputs:

- there are access points with NO DHCP server.  I would characterize them as "a switch with a radio".
  The Cisco WAP321 is an example I believe.

- there are access points with a built-in DHCP server.  So, "a switch with a radio with a DHCP server".
  The DLink DWL3200AP and DAP-2360 are examples.

- then there's the Router-as-an-Access-Point approach which is also "a switch with a radio with a DHCP server".
Just about any commodity router will fit this description (with the WAN not connected).

In the latter case we generally recommend that the router-as-an-access-point have DHCP turned OFF so it doesn't conflict with the network's (generally wired) DHCP server.

In the first case, there's no issue or question.

But, in the middle case, I'm reading that the access point will provide addresses with its DHCP to "wireless clients".  This implies there is isolation between the DHCP server and the ethernet port.  Is that true?
If so, does this mean that one might run two, isolated DHCP servers?  One for wired clients and one for wireless clients (assuming the simplest possible arrangement)?
It's a little hard for me to imagine that the wireless clients would be isolated from the "wired" DHCP server and still connect to everything else on the wired network.
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Fred MarshallPrincipalAsked:
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JohnBusiness Consultant (Owner)Commented:
Let me state what I do and is a bit different than you are surmising, most around DHCP.

I will take a good Wireless Router and hook it up this way:

1. Connect a LAN port on the Wireless Router to the network by an Ethernet cable.
2. Give the Wireless Router a static IP on the network.
3. Turn DCHP OFF on the Wireless Router.

Now it is an extension of your network.

If you wish to isolate the Wi-Fi, best to have VLAN capability in your network and hook up the Wi-Fi to a VLAN segment. This will isolate it better than just having two DHCP servers on one network.
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Fred MarshallPrincipalAuthor Commented:
John:  What you described is #3.  
The question is about isolation or non-isolation of DHCP services.
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JohnBusiness Consultant (Owner)Commented:
I would not use DHCP in a simple access point as I prefer a basic access point (#1) or a router (#2).  I do not like extra DHCP servers, and in number 2, it seems to me it would conflict. I just don't do this.
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Fred MarshallPrincipalAuthor Commented:
John:  Neither do I.  
The question is about how various devices actually work and not about what one would do.

Here is an example of something that might, in some circumstances be useful to know:

1) IF a wireless access point will only provide DHCP service to its own wireless clients
2) AND if it will isolate those clients from any "wired" DHCP server
THEN maybe using DHCP on that access point would be useful to some.

Useful how? one might ask:  If all this applies then there could be a separate wireless IP address subrange and one could tell if a client were a wireless client (or not) - in the best case barring any static addresses in that same range which could be forced by a client.

The documentation for the DLink devices implies the first part but isn't clear on the second part.
I don't know what other devices will do other than those that have no DHCP server.
And, of course there are those with no DHCP server at all.

NB that I said:
In the latter case we generally recommend that the router-as-an-access-point have DHCP turned OFF so it doesn't conflict with the network's (generally wired) DHCP server.
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JohnBusiness Consultant (Owner)Commented:
If the internal networks are connected, I see two issues:

1. If the wireless user assumes network access, they cannot if on a different DHCP range.
2. If the client assumes different DHCP range means security, it does not.

So I would turn DHCP OFF as you are also saying.
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Fred MarshallPrincipalAuthor Commented:
John:  That be as it may, it doesn't address the characteristics of access points.
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JohnBusiness Consultant (Owner)Commented:
Hmmm,  DHCP can be used or not used so I don't think it is an embedded characteristic of access points, just a setting which can be made. I am not sure what else we should say.
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Fred MarshallPrincipalAuthor Commented:
One could say that they know how the access point actually functions - that was the original question.
How one chooses to use them, with their varying characteristics, is another matter entirely.

For example:
1)  one cannot use an access point's DHCP server if it doesn't have one.  So this is not a case where DHCP can be used no matter what one may want.
2)  one might use an access point's DHCP server if it's the only DHCP server on the network.  That's how I use some of them.
3) one might want to do #2 even when there's another DHCP server on the network IF there's isolation of the services and each with separate lease address ranges.  (The DLink documentation suggests there's at least enough isolation to serve only its own wireless clients.  But this doesn't convince me that a wired DHCP server on the same network might overlap.  That is, the access point will only serve it's own wireless clients but a wired DHCP server might also be available to serve those same clients.  I'm not sure that isolating the latter is even possible - after all, the wireless clients are switched into the broader network).

I figured that someone might actually know how these things work in practice.
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Fred MarshallPrincipalAuthor Commented:
Here's an example of how this question might be addressed:

Q: With a DLink access point like a DWL-3200AP or a DAP-2360, the description says that the DHCP service provides addresses for the wireless clients.  OK.  I believe that.
But:
1) will the same access point DHCP service run over the network that the access point is connected to?  That is, if one wires a computer to that network (let's assume the network has no other DHCP server) then will that wired computer get DHCP service from the access point (i.e. "back" through the wire?).
2) If the access point service is turned ON and the connected network also has a DHCP service, might wireless clients on the access point get DHCP service OTHER THAN from the access point's own DHCP service? (i.e. "forward" off the wire?).
3) If the access point DHCP service is OFF and there is a DHCP server on the network already then WILL wireless clients on the access point  get DHCP service ? (i.e. "forward" off the wire?).  Presumably the answer to this question is Yes.  It's the only mode for some access points.
.
A:
1) Access point DHCP back onto the wire? Yes or No?  I have no idea.....
2) Access point DHCP ON and wired DHCP forward to wireless clients? Yes or No?  If (3) below is Yes then it seems there must be some blocking mechanism due to the ON state for this to be No.
3) Access point DHCP OFF and wired DHCP forward to wireless clients? Yes?
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Craig BeckCommented:
I tend to not particularly class routers with wireless capabilities as 'access points'.  Yes, they have an access point in them, but as you correctly identified, they act differently and are what I'd call "an exception".

A traditional access point is a layer-2 bridge.  I wouldn't particularly call it a switch, as traditionally they're not that intelligent.  I'd prefer to call an access point a hub rather than a switch.

When throwing DHCP into the equation you can have it a couple of ways.  The first way (forgetting routers here) is to simply provide DHCP over the wireless medium.  As you noted, Fred, this appears to be isolating the wired interface.  This is purely a firmware thing, as a bridge simply converts one medium to another.

The second way is to simply add a DHCP server to a bridge.  This, again, is firmware.  In this case the DHCP server can service clients on both interfaces.

It's usually a simple packet filter that either allows or blocks DHCP on the wired interface.  Usually the only way to know for sure how the DHCP server operates is to connect it up and test it.  I usually just connect a laptop to the wired interface of the access point and set the NIC to DHCP.
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Fred MarshallPrincipalAuthor Commented:
craigbeck:  Good point about switch vs. hub.
Yes, I understand this stuff is in firmware.  The question is about "what does it do?".

I tried it on a DLink DWL-3200AP and got these results:

1) Wireless clients get their address from the DWL-3200AP if DHCP is Enabled there.   Even if the local router DHCP was Enabled also did I get a lease from the router instead of the AP.  Not proof but likely the wired DHCP packets are blocked when DHCP is Enabled on the AP.

2) Wireless clients get their address from the local router if the DWL-3200AP DHCP is Disabled.

3) Wired clients do not get addresses at all if the router's DHCP is Disabled - which means the AP does not "serve" the wire.  Blocking in the opposite direction I suppose one could say.

The question remains, is this common behavior?  It does seem most reasonable.  
Does anyone know of APs which have controls over any of these behaviors?
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Craig BeckCommented:
All of that is common.  It's entirely dependent on how the firmware implements the bridge though.

Cisco APs with autonomous code can do it both ways depending on how you configure it.
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Fred MarshallPrincipalAuthor Commented:
craigbeck: "Both ways" as in ..... ?
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Craig BeckCommented:
Both ways, as in...

It can serve wireless clients only using its built-in DHCP server.

It can serve wireless AND wired clients using its built-in DHCP server.
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Fred MarshallPrincipalAuthor Commented:
It seems like we got somewhere here.  It seems to remain that one may need to test.  But as craigbeck says: "All of that is common".
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