VLSM
CiDR(Classless Inter Domain Routing)

class (IP address) A,B,C,D,E
classful routing : RIPv1, iGRP
< 사이에 EiGRP(iGRP에서, EiGRP를 알려면 classless도 알아야 한다.)
classless routing : OSPF, isis

major network
subnetwork

summary (auto summary)

IP Class

Class	High Order Bits	Start	End
Class A	0	0.0.0.0	127.255.255.255
Class B	10	128.0.0.0	192.255.255.255
Class C	110	192.0.0.0	223.255.255.255
Multicast	1110	224.0.0.0	239.255.255.255
Experimental	1111	240.0.0.0	255.255.255.255

http://penguin.dcs.bbk.ac.uk/academic/networks/network-layer/ip/index.php

가장 왼쪽 비트(High Order Bits)가 0, 10, 110, 1110 로 시작.
0~ : 50% (A Class) 0~128
10~ : 25% (B Class) 129~191
110~ : (C Class) 192~223
1110 : (D Class for multicast) 224~239
1111 : (E Class for experimental) 240~255

첫번째 옥텟으로 클래스를 알 수 있다.