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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" 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?

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Yikes. This thing isn't running an IOS version from before CIDR, was it? –  Christopher Karel Jun 20 '13 at 15:33
No, definitely not. It's a fairly new device, although you couldn't tell that from the web interface they created. ;-) –  Dustin Dauncey Jun 20 '13 at 16:02

2 Answers 2

up vote 1 down vote accepted

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

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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. –  Dustin Dauncey Jul 11 '13 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 '13 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.

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