Supernetting and Classless Interdomain Routing
With the recent growth of the Internet, it became clear to the Internet authorities that the class B network IDs would soon be depleted. For most organizations, a class C network ID does not contain enough host IDs and a class B network ID has enough bits to provide a flexible subnetting scheme within the organization.
The Internet authorities devised a new method of assigning network IDs to prevent the depletion of class B network IDs. Rather than assigning a class B network ID, InterNIC assigns a range of class C network IDs that contain enough network and host IDs for the organization's needs. This is known as supernetting. For example, rather than allocating a class B network ID to an organization that has up to 2,000 hosts, the InterNIC allocates a range of eight class C network IDs. Each class C network ID accommodates 254 hosts, for a total of 2,032 host IDs.
Although this technique helps conserve class B network IDs, it creates a new problem. Using conventional routing techniques, the routers on the Internet now must have eight class C network ID entries in their routing tables to route IP packets to the organization. To prevent Internet routers from becoming overwhelmed with routes, a technique called Classless Interdomain Routing (CIDR) is used to collapse multiple network ID entries into a single entry corresponding to all of the class C network IDs allocated to that organization.
Conceptually, CIDR creates the routing table entry: [Starting Network ID, count], where Starting Network ID is the first class C network ID and the count is the number of class C network IDs allocated. In practice, a supernetted subnet mask is used to convey the same information. To express the situation where eight class C network IDs are allocated starting with network ID 220.78.168.0:
Starting Network ID
220.78.168.0
11011100 01001110 10101000 00000000
Ending Network ID
220.78.175.0
11011100 01001110 10101111 00000000
Note that the first 21 bits (underlined) of all the above Class C network IDs are the same. The last three bits of the third octet vary from 000 to 111. The CIDR entry in the routing tables of the Internet routers becomes:
Network ID
Subnet Mask
Subnet Mask (binary)
220.78.168.0
255.255.248.0
11111111 11111111 11111000 0000000
In network prefix or CIDR notation, the CIDR entry is 220.78.168.0/21.
A block of addresses using CIDR is known as a CIDR block.
With the recent growth of the Internet, it became clear to the Internet authorities that the class B network IDs would soon be depleted. For most organizations, a class C network ID does not contain enough host IDs and a class B network ID has enough bits to provide a flexible subnetting scheme within the organization.
The Internet authorities devised a new method of assigning network IDs to prevent the depletion of class B network IDs. Rather than assigning a class B network ID, InterNIC assigns a range of class C network IDs that contain enough network and host IDs for the organization's needs. This is known as supernetting. For example, rather than allocating a class B network ID to an organization that has up to 2,000 hosts, the InterNIC allocates a range of eight class C network IDs. Each class C network ID accommodates 254 hosts, for a total of 2,032 host IDs.
Although this technique helps conserve class B network IDs, it creates a new problem. Using conventional routing techniques, the routers on the Internet now must have eight class C network ID entries in their routing tables to route IP packets to the organization. To prevent Internet routers from becoming overwhelmed with routes, a technique called Classless Interdomain Routing (CIDR) is used to collapse multiple network ID entries into a single entry corresponding to all of the class C network IDs allocated to that organization.
Conceptually, CIDR creates the routing table entry: [Starting Network ID, count], where Starting Network ID is the first class C network ID and the count is the number of class C network IDs allocated. In practice, a supernetted subnet mask is used to convey the same information. To express the situation where eight class C network IDs are allocated starting with network ID 220.78.168.0:
Starting Network ID
220.78.168.0
11011100 01001110 10101000 00000000
Ending Network ID
220.78.175.0
11011100 01001110 10101111 00000000
Note that the first 21 bits (underlined) of all the above Class C network IDs are the same. The last three bits of the third octet vary from 000 to 111. The CIDR entry in the routing tables of the Internet routers becomes:
Network ID
Subnet Mask
Subnet Mask (binary)
220.78.168.0
255.255.248.0
11111111 11111111 11111000 0000000
In network prefix or CIDR notation, the CIDR entry is 220.78.168.0/21.
A block of addresses using CIDR is known as a CIDR block.
No comments:
Post a Comment