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I'm using several AirPort Extremes to create a wireless network for a large amount of devices. One AirPort is advertised to support 50 devices, so I'm using 4 AirPorts currently for 200 devices.

My setup consists of 1 AirPort in DHCP and NAT mode, and the remaining AirPorts in Bridge mode.

Now, I'd like to extend the network capacity to 400-ish devices.

One issue I see is that AirPort Utility only supports setting DHCP ranges within a /24 subnet, meaning that there is a maximum of around 250 devices possible.

It is important that all devices share the same subnet, because the used application requires UDP broadcasts to communicate with other peers.

It is also important that roaming (i.e. moving around in the room and reconnecting to other base stations without disconnects) is possible. This works fine with my 4 AirPort setup so far.

What is the preferred way to create a larger than default subnet (e.g. /23) using Apple AirPort Extreme?

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The maximum number of addresses you can have on a /24 subnet is 256, so you are right on target with having 250 or so. This includes the IP for the gateway, and the broadcast, so, you are now down to 254.

For example, using a Class C subnet with a mask of /24 your range will be 

192.168.0.0 to 192.168.0.255

This is not a limitation of the Airport, but of the IP network itself. What you can do is change the netmask so it will allow more IP's within the given subnet:

255.255.254.0

Will allow a subnet with up to 508 clients.

192.168.0.1 to 192.168.1.255

Totally opinionated section....

If you are managing this many devices/clients on a network, Airport is NOT something I would use. I would look at getting "Enterprise Grade" networking gear like Ubiquiti UniFi Pro with much better management capabilities. They even have a "lite" version in which the all the controller software and config is centralized.

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  • Comments are not for extended discussion; this conversation has been moved to chat. – bmike♦ Oct 16 '16 at 15:01
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    Excuse me, but this is flat out wrong. A mask of 255.255.254.0 is equal to /23 and it is a single subnet because that's what a subnet is. Your answer claims that it somehow is two subnets. Also, having the IP addresses in a class C network area has absolutely nothing to do with the largest mask available. – oals Oct 16 '16 at 16:25
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    So basically the questioneer is asking 'how do I set the airport device to have a /23 subnet' and your response is 'you can't, just use two /24s' (though your basis for saying that is completely wrong.) – oals Oct 16 '16 at 16:26
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I would get an edge router and place your AirPort devices in bridge mode.

AirPort does a lot of things right - it doesn't have enough horsepower to manage more than 256 addresses - so let it do the things it does well like manage Bonjour sleep proxy and get a router to do routing and DNS/DHCP. You get bonus QOS and VPN features on these edge routers that will help you manage access in and out. That many clients will kill your network connection upstream if you don't have visibility and controls.

In my opinion, Apple chose wisely to not enable more clients than its hardware can manage. There's no use case where I can see you being happy with 200+ clients being routed by Apple gear.

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  • Will roaming and the broadcasts/multicasts work with an external DHCP? Is there a practical limit on how large a subnet can be before things start breaking? Will AirPort utility still work to configure the AirPorts even when I build a network with 8 of them? Will roaming still work even if I combine Apple APs with other vendor's? WIll configuration through AirPort utility still work? – Etan Oct 16 '16 at 14:22
  • @Etan You are asking all the right questions. I would encourage you to keep all your clients that care about multicast on the same /24 - much better to set up a different /21 for all the low performance needs. If you really need true control of 300 clients to include multicast - you should probably budget for AeroHive or other centrally managed network. $5k to $20k of hardware and software would be a good estimate on costs there to do all of what you ask above with no restrictions. – bmike♦ Oct 16 '16 at 14:40
  • $5k to $20k is quite a broad range ^^ Is there a chat option here so we could discuss this more deeply (given that you have some time available) – Etan Oct 16 '16 at 14:47

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