Network | CIDR

No category (now)

Classless Inter-Domain Routing (Classless inter-domain Routing, CIDR) is a method of classifying IP addresses for assigning IP addresses to users and for efficiently routing IP packets over the Internet.

CIDR uses the variable eldest-son netmask (vlsm,variable Length Subnet masking) to assign IP addresses according to individual needs, rather than following a rule that is a network-wide convention. Therefore, the division of the network/host can be done anywhere within the address. This division can be recursive, that is, by increasing the number of mask bits, a portion of the address is continued to be divided into smaller parts. The entire internet is now using CIDR/VLSM network addresses. In addition, CIDR is used in other ways, especially in large private networks.

Another benefit of CIDR is the ability to prefix route aggregation. For example, 16 of the original Class C (/24) networks can now be aggregated together, showing A/20 network (if the first 20 bits of those networks are the same). Two aligned/20 networks can be further aggregated to/19, and so on. This effectively reduces the number of networks to be externally displayed, prevents ' routing table explosions ', and also curbs the further expansion of the Internet.

Classification Network (past)

Classful addressing

Most Web sites are too big for the network address of Class C, so they all get the address of Class B. With the rapid development of the Internet, the address of Class B (a total of 214) has been rapidly reduced. The classification network was superseded by Classless Inter-domain routing in 1993 to address this problem.

Classification	Prefix code	Start Address	End Address	corresponding CIDR modifiers	Default Subnet mask
class A Address	0	0.0.0.0	127.255.255.255	/8	255.0.0.0
class B Address	10	128.0.0.0	191.255.255.255	/16	255.255.0.0
class C Address	110	192.0.0.0	223.255.255.255	/24	255.255.255.0
class D address ( multicast)	1110	224.0.0.0	239.255.255.255	/4	Not defined
class E address (reserved)	1111	240.0.0.0	255.255.255.255	/4	Not defined