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.
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.