Is there a risk in letting DHCP set the IP address of an access point?

Question

To get better Wi-Fi in the attic, I have installed an access point connected by cable to my downstairs router. I have done this before, setting the IP address of the AP to a fixed address outside the DHCP address range of the router (why? because some random guide on the internet said so).

However, this time round, I forgot to set the IP address of the AP to a fixed value. To my surprise, this seems to work perfectly fine: the AP attained an IP address through DHCP on the router, and all is well.

Is there a good reason to set the IP address of the AP to a fixed value? If yes, what are the risks of not doing so?

Answer 1

Access points are bridges; the client does not actually talk them at IP layer. Indeed the AP might not even have an IP address visible to the client at all, because it's only needed for administration (i.e. accessing the AP's control panel).

For example, enterprise multi-SSID access points might offer 2–3 SSIDs with distinct subnets but still have only one IP address, and often it's in a 'management' subnet that's completely separate from any of the SSIDs.

So there's nothing wrong with using DHCP for APs – though I would recommend giving such devices a fixed address at DHCP server level (using "reservations" aka "static leases"), just so that you could find it later.

Answer 2

TL;DR: If a node is static, i.e. is always present on your network, use static addressing. If nodes enter and leave your network dynamically, use dynamic addressing (DHCP). That is true at least, if you don't have a name system.

Long version: Using addresses supplied via DHCP is not a problem per se.

It might pose a problem, if you later need to know the address for some purpose, but don't have an easy means of retrieving it.

A well behaved DHCP client will actually use the address leased from the DHCP server. It seems reasonably safe to assume, that this is the case here. It would thus be safe to assume, that knowledge of the information in the DHCP server's lease table will be enough to easily get the current address of your AP.

If the client behaves badly, you now have several additional points of failure (not exhaustive):

1. DHCP client (AP) got a lease, but doesn't use it, due to some failure. It is unknowable, which address the device is using, without listening in on the network. - If you're lucky, the device listens on a predefined backup-address, and you will be able to reach it using that. - If you're not, you might have to access it via a different means, or reboot it, to force a new DHCP request. - Even worse: If this does not work, you might need to reset the device.
2. DHCP client (AP) didn't get a lease. Maybe the DHCP server was not available. It might now use a fallback address unreachable on your network. - Same as above: You might be lucky, but you as well might not be.

Side note: Just to make sure, that you're not misunderstanding how DHCP works. (You wrote in your question about "DHCP setting the address"): DHCP is a system for address space management, only! It makes sure, that in a well configured network with only well-behaved clients, no address will be used by more than one client at a time. - It does so by giving the DHCP server the sole authority of assigning addresses by leasing them out to requesting clients. - The client then "sets" the address, by communicating to the network that it will be using the address and by subsequently listening to traffic addressed to it.