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I've just expanded our subnet at work from /24 to /22 now that we've started running out of addresses. For ease of transition, I left all statically-assigned IPs at their original addresses and moved the DHCP pool to a new block of 254 addresses. From everything I've read and understand, it should be that simple, but instead everything broke...

The new pool works, but anything with a MAC-reserved IP either gets a generic IP from the pool, or no address at all (DHCP fails). I got desperate enough to try flushing the ARP cache and DHCP leases and rebooting the device with no effect. To make this change, all I did is edit the X2 interface to have netmask 255.255.252.0 instead of 255.255.255.0. Then added a Dynamic DHCP scope to the new block of addresses. Is there anything I could be missing? This is so broken I feel like I must've missed something big. Thanks for your help, and Happy New Year.

EDIT: Running SonicOS Enhanced 6.2.7.1-23n on TZ 500. Also, if I manually assign the addresses that DHCP should be assigning to the reserved PCs, they can communicate with other machines on the network.

Screenshot of DHCP Setup: Sonicwall DHCP Screenshot

Sonicwall DHCP Config Screenshot Sonicwall Interface Config

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  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin
    Apr 19 '19 at 15:01
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You probably need to change the Subnet Mask on each static DHCP entry to match the interface:

Sonicwall Static DHCP entries

When you add a new entry, you can choose to populate it from an exising interface (documentation). However, it does not dynamically update the entries when the interface is changed, you have to do that by hand.

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  • I looked at the entries and they all appear to have updated. The 'X2 Subnet' object updated itself and all the static entries show "255.255.252.0". I was actually impressed that seemed to work automatically until I realized so many things actually appeared to be broken. Dec 31 '18 at 18:41
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OK I think I've got it working, but only somewhat sure what did it. Through a combination of flushing the ARP Cache and Deleting DHCP leases, I got it to assign the correct addresses. There was one hold-out machine that kept receiving a DHCP pool address rather than it's statically-assigned one, and I was able to get it working by deleting the DHCP lease, then flushing its entry from the ARP Cache, then finally renewing the IP.

My best guess is that changing the DHCP scope settings took a while to propagate due to the ARP Cache and existing leases. I also think I had "Enable DHCP Server Persistence" enabled when I power-cycled the device. Otherwise it may have been fixed then.

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