NAT and large DHCP pools

I have a large number of WiFi devices that need a onetime set-up and then will be sent out to users. At any given time we have 30 or 40 devices being set-up and sometimes need to set-up several hundred a week.

I'm looking for a router with NAT and DHCP that's happy to see devices only once and either release IP addresses quickly or support a large pool. Any ideas?

We did have a Google WiFi but it's too clever! It tries to remember all the MAC addresses so you can name them and set access profiles. After 255 MAC addresses it seems to sulk.

Get a router that will allow a larger than 255 LAN IP range - which I cannot answer, but have you tried doing that?

Sounds to me your running out of IP addresses because of the IP address lease time value which may not be configurable.

A lot of routers have a subnet mask of 255.255.255.0 (aka CIDR /24) which means you get 255 IP addresses (e.g. 192.168.0.1-192.168.0.255), if you can find a router that supports a subnet mask of 255.255.254.0 (aka CIDR /23) you will get 510 IP addresses (e.g. 192.168.0.1-192.168.1.255) or if that's not enough try a router that supports 255.255.252.0 (aka CIDR /22) then you will get 1020 IP addresses (e.g. 192.168.0.1-192.168.3.255). Alternatively try to find a router that supports configurable lease times for the LAN / WLAN and set it to a very short period.

Sorry cannot recommend any products as I'm retired now and not up to date with what's now available.

Edited by deleted (Sat 12-Jan-19 16:02:40)

Set a short lease time on the DHCP server. Simples.

If you set the lease time to 60 mins that would give "up to" 24 x 255 devices per day.
Devices auto renew lease if needed so this shouldn't be an issue.

In reply to a post by bsdnazz:

I'm looking for a router with NAT and DHCP that's happy to see devices only once and either release IP addresses quickly or support a large pool. Any ideas?

Many routers let you configure the DHCP parameters. Setting a lease time of 30 minutes would solve your problem.

(Some people incorrectly think this will cut your machines off every 30mins, but that is not how DHCP works).

Does anyone know of a router that does let me set a short DHCP lease time and ideally a bigger than 256 address class C network?

Take a look at the Ubiquiti devices. The Edgerouters and USG's support both those requirements. Some Draytek's do too.

ASUS routers let you configure these DHCP settings:
* IP Pool Starting Address
* IP Pool Ending Address
* Lease time (in seconds)
* Default Gateway
* DNS servers

If you find a router that allows you to up the amount of IP addresses and reduce the lease time period you also need to ensure it supports the number of concurrent connections you require.

Just for the sake of mentioning it - the EE SuperHub I have allows me to set the lease time between 1 hour and 21 days.

Do the IP addresses have to be in the Class C 192.168.???.??? range? What router are you currently using?

As for routers, both of which I use, the DrayTek Vigor2862 family and Ubiquiti EdgeRouter X can allow large DHCP pools and short DHCP lease times.

For the EdgeRouter X you will need an external WiFi AP. If you get the appropriate member of the Vigor2862 family the WiFi AP will be built-in.

But how big a pool can you actually have? I can set the pool starting and ending addresses on my Zyxel router but I found that the first 24 bits of the IP addresses have to be the same. The router changes what I enter and its own IP address to ensure that it conforms.

Draytek 2862 and 2926 certainly do on lan 1-3. Up to 1022 per subnet.

In reply to a post by Michael_Chare:

But how big a pool can you actually have? I can set the pool starting and ending addresses on my Zyxel router but I found that the first 24 bits of the IP addresses have to be the same. The router changes what I enter and its own IP address to ensure that it conforms.

I don't know as I've not tried. Given the router lets you change its own IP and subnet mast, it is possible it supports larger subnets than /24.

The ASUS range certainly supports hundreds of thousands of connections through the NAT table. (300,000 or more).

I've been puzzled since this thread was started.

If the devices are being sent out after being configured, at a guess something like CCTV cameras or associated kit like "Ring" remote video doorbells, the problem with local DHCP/NAT seems to be the storing of MAC addresses and the table filling up. Not the number of IP addresses itself.

I've just googled about MAC aging time as it isn't something I have seen as settable on my domestic level routers. Not that I've been looking.

A Juniper article. Cisco has similar.

Am I barking up the wrong tree?

In reply to a post by RobertoS:

Am I barking up the wrong tree?

Sort of, you've conflicted two topics, because home routers have in one box multiple functions, network routing, NAT, DHCP server, DNS proxy, network switch etc. Each one of these handles things independently despite being inside the one box.

The Juniper article is about switching, where the MAC table associates a hardware device with an IP address in able for the switch to work. Generally this is not a problem as switches can remember hundreds of addresses and IPs and the switch ages out ones not seen. Juniper switches don't generally do DHCP (although they are insanely flexible and you probably could).

The problem in this thread is related to DHCP, which separately holds a list of MAC addresses that request an IP. When the pool of IPs is exhausted, no new devices (new MAC addresses) can get issued an IP.

The answer is to reduce the "lease duration" so that the DHCP server releases addresses back to the pool quickly.

Others have suggested enlarging the network subnet, so that more than 250 addresses can exist in the DHCP pool. Something that is very easy if you use Windows, Linux or another OS to run DHCP - is not obvious if you are trying to use home grade routers.

Edited by jchamier (Sun 13-Jan-19 16:51:36)

Ah, thanks .

It still comes down to the releasing though, as mentioned in the OP. Given the "hundreds per week", maybe the answer is simply a weekly or even daily factory reset of his router and re-loading of a pre-saved config?

OP here.

Thanks for the suggestions. I'll look into the Draytek 2862 and ER-X devices.

I've been looking to increase the pool size and/or reduce the lease duration.

I want to avoid a daily reset to clear DHCP allocation tables.

Good luck bsdnazz, hope you find a solution that works for you

If he can reduce the lease time to 15 or 30 minutes, the problem will go away.

A reboot probably doesn�t clear the DHCP tables, reloading configuration might but will depend on the make.

Have to test.

Yep. But I did say a factory reset and re-load, not a reboot .

Edited by RobertoS (Sun 13-Jan-19 21:26:07)

In reply to a post by RobertoS:

Yep. But I did say a factory reset and re-load, not a reboot .

Potentially quite a lot of work depending on how many devices. Much easier to reduce the lease time.

I have seen DHCP servers in companies where they had a lot of mobile staff (laptops and phones/tablets coming and going) where they did not have enough IP addresses, so the lease time ended up at 10 minutes.

It worked.

Ah, we are at cross-purposes. I was thinking of a dedicated device setup for despatch router. Not a traffic routing one.

I assumed they�d need internet, but nothing to stop plugging a second into a first and using second as you described.

I just don�t think it would be needed to go to the effort you describe.

I've bought and installed a Draytek 2862 and so far all looks good.

On the lan side I've configured the network at 192.168.1.1/22 which allows for 1024 IP addresses ( less 0 and 3.255 ).

The Draytek DHCP server is running on the network up it seems to only want let me have a pool size of 752 rather than nearly 1024. 752 is fine for my needs. Also, I can set the lease time to 120 seconds which means the IP addresses will be freed up quickly.

Hope you still got sorted.

For others who visit this after:

It's poor practice to have massive subnets (e.g /16) with DHCP - hence it most likely will be difficult to find a router with a large subnet capable DHCP address size. Subnets like a /23 or /22 should be ok for a decent SOHO router.

I'd recommend a few things:

- Splitting up your networks into smaller subnets.

- Set your lease time a little above what you expect commissioning time of the devices you're setting up. e.g. If it takes your about an hour to commission a device then set lease timer to 4 hours or something. I'm not sure the rhythm you have on setting up the devices - but take that concept. Meaning, by the time a DHCP lease expires you'll be well finished commissioning the device(s).

- Clear ARP each morning when you come in - might be an option. Again depends on how you're chain commissioning devices.

- Investigate writing a script to automate refresh. Either - script on the device you're setting up.. OR a script to test IP ranges for ping response, and if failed response, runs a command on a router to clear that ARP entry.


I personally would see about adding/script to release its IP when done.

For example - if commissioning hundreds of laptops - I'd have a USB pen with all my software I was installing then a final batch script on it to run "ipconfig /release" in command prompt - which will tell the DHCP server that the IP is now available for another client.