1

Hi,

I have a 0/21 class B network. Currently our DHCP scope is from 10.0.3.1 - 10.0.3.254. I would like to expand the range to 10.0.2.1 - 10.0.3.254. Is it that simple to adjust the range from the screenshot below? Or there anything else I need to do? Are there any repercussions might come about if I proceed with this?

DHCP.png

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2

I don’t believe you can change the subnet mask there. If you were to do this, you would most likely need to delete that scope and then create a new one with your desired settings.

As far as possible repercussions:

  • Do you have any DHCP reservations? If so I would recommend exporting / backing those up so you can easily recreate them.

  • I would also recommend recording your DHCP scope options before doing so (e.g. DNS server(s), NTP server(s), any special options for WLAN / VOIP / etc)

  • Do you have any network / ACL / firewall / content filter / endpoint or network protection software / monitoring systems / router configurations that are set to “care” about that subnet? (e.g. routes, firewall rules, ACLs, etc)

  • If you have the subnet defined in Active Directory Sites and Services, it would need to be updated there as well

  • Endpoints on that network using DHCP would obviously need to be rebooted or ipconfig /release and ipconfig /renew after the changes to make sure they pickup addresses in the new scope and can communicate

  • Do you have any other subnets active in your network that may overlap with your planned subnet?

  • If you have any servers, devices, endpoints, etc. with static TCP/IP settings, they’ll have to be manually updated.

  • I would recommend reviewing your network documentation, notes, etc. just to make sure about any other considerations.

3

I will not be changing my subnet. It will remain the same. I think all I have to do is to change the 3rd octet in “Start IP address” to 2. Please look at the screenshot below.

Is that all I have to do?

DHCP1.png

4

Ah… I need to pay better attention to detail.

In that case, yes that’s all you would need to do because that subnet goes from 10.0.0.1 - 10.0.7.254.
Still worth checking configurations of any devices / systems that were previously referencing the smaller range for any reason.

5

You might want to restart the DHCP Server service (or restart it via the DHCP MMC) after making that change.

6

Agreed, should work fine.

I would exclude 10.0.2.255 and 10.0.3.0 though — some devices will puke on those addresses even though they are legitimate in the subnet.

For the same reason, I would avoid assigning ANY address ending in .0 or .255 to any device, either statically or by DHCP.

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7

Restarting the Windows DHCP Server service isn’t necessary for changes to take effect, HOWEVER changes aren’t written to the DHCP database until the service is stopped. If your server crashes or restarts unexpectedly, changes will be lost. That’s why it’s always a good practice to restart the service after making changes.

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8

Change the scope 10.0.2.1 - 10.0.3.254 and the mask 255.255.254.0/23 is enought to the desired range.

Your broadcast address will be 10.0.3.255

9

What devices? This myth has been around for decades and may have been true a very long time ago.

The only time I’ve seen an issue is with software, not hardware. When a programmer decides that he’s smarter than I am. In netscan.exe, for example, if you set a scan range at .0 to .255, he assumes you’re a dope and skips those two end addresses. But if you can from 1.0 to 2.255, he includes the 1.255 and 2.0 addresses just find.

I have very few /24 ranges and I always use the .0s and .255s in the middle. I’ve never had a device puke, but I have seen people melt down over it.

10

Thank you everyone for the help.

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