Explain CIDR addressing scheme.
1 Answer

i. New system calls for eliminating the notion of address classes entirely, creating a new classless addressing scheme, sometimes called Classless Inter-Domain Routing (CIDR).

ii. Classful IP addressing uses hierarchy to arrange the address space in several discrete classes of addresses that correspond to networks of different sizes.

iii. CIDR IP addressing uses a variable length prefix and a subnet mask to represent networks at a finer granularity of network size.

iv. In doing so, CIDR addressing increases the utilization of the address space. When combined with address allocation policies that aggregate routes, CIDR makes it possible to reduce the size of the routing tables required in router.

v. As the name implies, classless addressing completely eliminates the prior notions of classes. There are no more Class A, B, C blocks that are divided by the first few bits of the address.

vi. Instead, under CIDR, all Internet blocks can be of arbitrary size. Instead of having all networks use 8 (Class A), 16 (Class B) or 24 (Class C) bits for the network ID, we can have large networks with, say, 13 bits for the network ID (leaving 19 bits for the host ID), or very small ones that use 28 bits for the network ID (only 4 bits for the host ID).

  • Classless Interdomain Routing :

    i. CIDR is an addressing scheme that uses all available addresses more efficiently.

    ii. The CIDR address scheme is a more flexible address allocation scheme which allows for host or subnet (subnetwork) address ranges of any length (rather than the strict 8-bit, 16-bit or 24-bit host and subnetwork address lengths of the classful address scheme).

    iii. When an address is assigned, the network and host IDs don’t always begin and end on octet boundaries according to the IP address class, Instead the network address and host ID can be almost any number of bits.

    iv. Example: If company requires only 30 host addresses, ISP might give you the network address In a 32-bit IP address, the /27 means the first 27 bits of the IP address designate the network ID and the last 5 bits designate the host ID.

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Fig6: Classful IP Addressing scheme

It is useful to commence with an example from the classful address scheme to explain the idea of the subnet-mask, and we return to Figure6.

The example class C address range shown in Figure6 and Figure 7 is the address range including all the addresses between and

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Fig7: CIDR Example

v. The host address (or subnet address) corresponds to the last 8-bits of the address (i.e., the value between 0 and 255 at the end). The subnet mask is a series of 24 bits of binary value '1', followed by 8 bits of binary value '0'.

vi. By comparing the subnet mask with the full IP address, the subnet address can be obtained by ignoring all the bits of the IP address which align with a value '1' in the subnet-mask.

vii. Whereas in the classful addressing scheme a whole address range could be identified by means of the gateway address, it is necessary in the case of classless addresses to specify the gateway address and the subnet mask.

  • Subnet Masks:

    i. IP uses an address’s subnet mask to determine which part of the address identifies the network portion and which part identifies the host portion, Subnet masks are 32-bit numbers in dotted decimal format.

    ii. Default subnet mask for Class A is

    iii. Default subnet mask for Class B is

    iv. Default subnet mask for Class C is

    v. Example: If a computer has the IP address and the subnet mask is then the network portion is 153.92 and the host portion is 100.10.

    vi. Using the same address of but with a subnet mask of, the network portion is now 153.92.100 and the host portion is 10 by altering the subnet mask, the network id has been altered.

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vii. The subnet mask can be denoted in one of two ways: either as a number similar to an IP-address comprising four decimal numbers equivalent to the 'value' of the subnet mask or as a 'slash character' (/) and a decimal number equal to the number of 1’s in the mask: thus in our example /24.

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