Why can't IP addresses start with 0 or 255?

The IP address 0.0.0.0 is used as a wildcard to bind to all addresses. The IP address 255.255.255.255 is used to indicate a local broadcast.

Because this decision was part of the classful network design that predates CIDR, to reserve these specific IP addresses alone, special rules would have had to have been created for the /8's that contained them. Since some /8's were going to remain reserved anyway, it made sense to reserve these rather than creating additional rules so they could be used.

Because they are reserved for multicast and broadcast addresses

The RFC specifically has set them aside as "Reserved"

They are valid, but just aren't usable

0.0.0.0/8 and 255.0.0.0/8 are both reserved by IANA in 1981. The former block is for self-identification (based on RFC 6890) and the latter is reserved for "Future use"; formally Class E.

Addresses starting with 240 or a higher number have not been allocated and should not be used, apart from 255.255.255.255, which is used for "limited broadcast" on a local network. See (RFC 919) and (RFC 922) for more.

Information from: https://myip.ms/info/whois/0.0.0.0 https://myip.ms/info/whois/255.0.0.0 https://tools.ietf.org/html/draft-wilson-class-e-01