Invalid subnet masks

[Matt Churchyard]

On 2015-2-11, at 09:59, Matt Churchyard <matt.churchyard at userve.net> wrote:
>> Just been helping someone on the forums who appears to have configured their network interface incorrectly. It looks like they've assigned 250.250.250.0 as the netmask.

> that's not invalid. The netmask is a mask and not a prefix like in IPv6.

> We could warn when people configure netmasks that are not contiguous prefixes (which is the usual practice), but such configurations need to remain allowed.

>Lars

I appreciate that it might be 'valid' as a binary mask, but I'm struggling to find any documentation anywhere that actually suggests that it's valid as a network configuration. The entire modern CIDR notation, and all the routing system & hardware built around it (that shows networks in CIDR form and will collapse routes) has no way of dealing with these subnets.

Are there actually valid use cases for these types of network?
I'm learning towards the opinion that they should be rejected unless the user specifically overrides it (with something like an ifconfig flag or sysctl). Although having said that, it's not really doing any damage letting people get their netmasks wrong. However, as I mentioned in my first email, Windows 8.1 (and I've now tested Server 2012 which is fairly common in enterprise globally...) will not allow them.