Final answer:
The x most significant bits of an IPv4 address a.b.c.d/x represent the network portion of the address, as part of the CIDR notation used for IP address allocation.
Step-by-step explanation:
The x most significant bits of an address in the form a.b.c.d/x constitute the network portion of the IP address. This notation is a part of CIDR (Classless Inter-Domain Routing) notation which allows for more efficient allocation of IP addresses than traditional classful networking. The network portion identifies the specific network to which the host belongs. The remaining bits after the network portion constitute the host portion, which uniquely identifies the device within the network.