Classless Inter-Domain Routing

A method of dividing IPv4 addresses into classes.

Overview

Class	Most-significant bits	Network prefix size (bits)	Host identifier size (bits)	Address range
A	0	8	24	0.0.0.0–127.255.255.255
B	10	16	16	128.0.0.0–191.255.255.255
C	110	24	8	192.0.0.0–223.255.255.255
D(multicast) E(reserved)	1110 1111	–	–	224.0.0.0–255.255.255.255

Classes

https://www.meridianoutpost.com/resources/articles/IP-classes.php

Class A

• Subnet Mask 255.0.0.0
• Network ID first octet
• Host ID last 3 octets
• Public IP range 0.0.0.0 - 127.255.255.255
• Private IP range 10.0.0.0 - 10.255.255.255 or 10.*.*.*

Class B

• Subnet Mask 255.255.0.0
• Network ID first 2 octets
• Host ID last 2 octets
• Public IP range 128.0.0.0 - 191.255.0.0
• Private IP range 172.16.0.0 - 172.31.255.255 or 172.16-31.*.*

Class C

• Subnet Mask 255.255.255.0
• Network ID first 3 octets
• Host ID last octet
• Public IP range 192.0.0.0 - 223.255.255.0
• Private IP range 192.168.0.0-192.168.255.255 or 192.168.*.*
• Special Loopback IP range 127.0.0.0 - 127.255.255.255 or 127.*.*.*