Zero in the last octet is problematic if you want to be generally reachable on the internet. We used x.y.z.0 as a vip at fb for a while, but learned the hard way that some people can't reach those ips.

Some additional info from RIPE: https://labs.ripe.net/Members/stephane_bortzmeyer/all-ip-add...

It's amazing how many stupid assumptions on the part of some people create these extraordinarily difficult situations to navigate.

It's a valid IP address, but it's the 'network address'. 'usable' range is 1-254. That's my understanding anyway?

That's not correct. Not every subnet is a /24. Further, you don't necessarily need a network and broadcast address for a subnet. That's a local issue. See IETF RFC 3021 for one example of not having a network and broadcast address.

it's the network address only if you define your network as a /24 . if the mask is different it could be a completely valid address.

0.0.0.0 is a "network" address no matter how many bits are in the netmask. Having said that, there really isn't a use of "network" addresses (especially as distinct from broadcast), apart from the convention being baked into basically everything.

:facepalm: right, of course, makes sense. Thanks.

Or anything smaller than a /24.