In Classful addressing scheme we have network that belongs to particular class. Network ID is the part that is defined by class id (A,B,C) with number of bits follows the class (either 8, 16, 24). If not divided, network has 2 parts: network and host ids based on class and default mask. If such network is divided into smaller parts (aka subnetted), we have created a new part called subnet id. Subnet id is between network and host parts and is created from borrowing the bits from host part. Default mask becomes now subnet mask. All 3 parts still must have 32 bits in total.
since classless address format is one of topics of CCNA exam, we must differentiate between classful and classless addressing format schemes. Here are official definitions from CCNA book by W. Odom.
Classless addressing: The concept that an IPv4 address has two parts—the prefix part plus the host part—as defined by the mask, with no consideration of the class (A, B, or C).
Classful addressing: The concept that an IPv4 address has three parts—network, subnet, and host—as defined by the mask and Class A, B, and C rules.
Subnet number: Also called the subnet ID or subnet address, this number identifies the subnet. It is the numerically smallest number in the subnet. It cannot be as an IP address by a host.
Regards, ML
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