Classful IP Addressing

---

Classless addresses are IP network and subnet IDs that are defined by network masks.

• address classes derive from the earliest form of IP
  • When first defined, IP did not include the concept of subnet masks
  • Hosts would identify the network ID just by using the address class

Classful addressing allocates a network ID based on the first octet of IP address.

• employed in 1980s before use of netmasks was developed
• Class A network addresses
  • 16 million+ hosts
  • 126 networks
• Class B network addresses
  • 65,000 hosts
  • 16,000 networks
• Class C network addresses
  • 254 hosts
  • 2 million+ addresses

Can identify the class from the first octet:

First Octet	Class
1–126	Class A
128–191	Class B
192–223	Class C

Reserved Class A IP Addresses

• These are part of Class A but reserved for special use cases:
  • 0.0.0.0/8 means “this” network
  • 127.0.0.0/8 is used for loopback addressing

Default Masks

• Classes are often used as names for netmasks that align to the whole octet boundaries
  • align precisely with octet boundaries
  • referred to as default masks

Class	Dotted Decimal Mask	Network Prefix	Binary Mask
A	255.0.0.0	/8	11111111 00000000 00000000 00000000
B	255.255.0.0	/16	11111111 11111111 00000000 00000000
C	255.255.255.0	/24	11111111 11111111 11111111 00000000