I'm a Sysadmin for a tech company. We have a Cisco RV180 gateway device. We are at our breaking point for IP addresses, so we are changing from a /24 to a /23 subnet to allow for double the devices.

I'm trying to make this change on the Cisco RV180, but when I change only the subnet mask from a address to a address, it provides the following error and refuses to save changes: "Invalid subnet mask. It should be 255 for given class of IP address at 255.255.xxx.0." Our LAN IP address is for the device, by the way.

Why won't it let me change to a subnet? Is it because it's forcing me to use classful scheme versus CIDR? I'm just surprised more than anything that it won't accept the new subnet mask. Any help here would be appreciated. I feel like perhaps I'm just overlooking something totally obvious.

If this isn't possible for some reason... any suggestions on what the next move would be so I can get more IP addresses in our LAN? Should I jump to something else?

  • Yikes. This thing isn't running an IOS version from before CIDR, was it? Jun 20, 2013 at 15:33
  • No, definitely not. It's a fairly new device, although you couldn't tell that from the web interface they created. ;-) Jun 20, 2013 at 16:02

2 Answers 2


Cisco tends to view CIDR as a means to subnet historically classful networks, not as a means to aggregate them.

In other words, this device will allow you to subnet a Class A or Class B network (e.g. or into the /23 that you want, but it won't let you combine two Class C networks, and, into

  • 1
    Makes sense. Thank you for your response. (Sorry for my delayed review of this question to follow-up again) I did setup a support case with Cisco and they confirmed after their own lab testing that it does not allow what we'd like. It's a shame really, for an otherwise solid router it has this incredibly useless arbitrary limitation. Jul 11, 2013 at 17:22
  • This is one of the reasons why we still teach new CIS students about the historically classful addressing scheme.
    – Skyhawk
    Jul 16, 2013 at 3:23

Looks like this is a known issue. It also appears that Miles is correct, in that you could create a <24 bit subnet mask with IPs outside of the usual Class C designation.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.