Is This a Valid Subnet?

I have recently learned that 10.0.0.0, Mask 255.255.255.0, is NOT a valid subnet.

I know it is NOT ROUTABLE over the internet. The problem seems to be that 10.0.0.x with mask 255.255.255.0 is not valid because of the 0.0 part. I read that the subnet cannot contain all 0’s or all 1’s. This seems to violate that rule.

I have used it for over a year with no problems. Is my new information correct?

If so, what problems might I run into?

Thanks so much,

Mark