Class-based subnetting architecture dividing addresses into three different classes:

Class A - 8 bits for network, 24 bits for host

• Subnet mask: 255.0.0.0
• First octet: 0-127 Class B - 16 bits for network, 16 bits for host
• Subnet mask: 255.255.0.0
• First octet: 128-191 Class C - 24 bits for network, 8 bits for host
• Subnet mask: 255.255.255.0
• First octet: 192-223

Classful subnetting is not formally used but used as a starting point and in conversation,

Class D (multicast) Class E (reserved)

Parts of a subnet

• Network address (first IP address of the subnet)
• First usable host address (one higher than network address)
• Network broadcast address (last IP address of the subnet)
• Last usable host address (one lower than network broadcast address)