IP Addressing is Classy

The IP protocol  has an addressing scheme divided into Classes

Class A

• designated for networks with many hosts, subnet mask of /8 (255.0.0.0)
• the high-order bit is always a 0 for Class A
• Valid networks are 1.0.0.0 thru 126.0.0.0 /8
• This allows for 126 networks with 16, 777,214 hosts per network

• 0.0.0.0 /8 is not a valid address and represents "this network"
• 127.0.0.0 /8 is not valid and is used as the loopback address for testing the local PC

Class B

• designated for medium to large-sized networks, 
• default subnet mast of /16
• the two high-order bits are always set to binary 1 0
• Valid addresses range from 128.0.0.0 to 191.255.0.0 /16
• The allows for 16,384 networks and 65, 534 hosts per network

Class C

• Designated for smaller networks with default subnet of /24
• the three high-order bits are always set to binary 1 1 0
• Valid address range from 192.0.0.0 to 233.255.255.0 /24

Private Addresses - Non-Public Internet Routable Addresses

• Class A: 10.0.0.0  to 10.255.255.255
• Class B: 172.16.0.0  to 172.31.255.255
• Class C: 192.168.0.0  to 192.168.255.255

Class D  -  IP Multicast Address

• The four high-order bits are always set to binary 1 1 1 0
• These addresses are not allocated to hosts and have no default sub-mask
• Valid addresses from 244.0.0.0  to 239.255.255.255

Class E     - Experimental and reserved addresses for future use

• High-order bits are set to 1 1 1 1
• Addresses are not allocated to hosts so no default subnet mask
• Addresses range from 240.0.0.0  to 255.255.255.255
• 255.255.255.255 is the broadcast address for 'this network'

Summary

CIDR - Classless Inter-Domain Routing    ( SUBNETTING) 

• CIDER removes the fixed /8, /16, and /24 requirements for address Classes
• The Classes can now be split or "subnetted" into smaller networks.
• ex 175.10.10.0 /20
• CIDER allows ROUTE SUMMARISATION