CIDR

eng_ahmedas

Hi All
I wish u r all in very good health..i wish u could help me answer those 2 questions:
1) What is CIDR ???
2) Why CIDR only used with classless routing protocols (as RIP V2 and OSPF) ???

Thanks in advance

EdTheLad

The original routing protocols were classful i.e. RIPv1 and IGRP.This means the subnet mask is not sent in the update.Since the update doesnt send the mask the ip address in the update will take the mask from the receiving interface if it belongs to the same major network.If it doesnt belong to the same major network as the receiving interface it will be summarized to its classful boundary.
Also all major network addresses must have the same network mask for consistency, if not,the update will not be forwarded by the receiving router.

Next came the classless protocols, these send their mask in the update allowing for vlsm, this means the major network can have varying length masks throughout the network i.e. 192.168.1.1/28, this has a major network of 192.168.1.0 but it can be variably subnetted with /28,/29,/30 as you like throughout the network once you dont overlap on ip addresses.

Next comes CIDR, above you have the network 192.168.1.0/28 this has a
major network boundary 192.168.1.0 due to being a class c, i.e. 24 bits reserved.As ive shown with vlsm you can subnet with masks greater than /24 i.e. /25,/26 etc... if you move the mask to the left of the major network boundardy i.e. /23,/22,/21 etc its a supernet, this is whats know as CIDR.

Hope you get it as maybe i rambled a little.

agustinchernitsky

if you move the mask to the left of the major network boundardy i.e. /23,/22,/21 etc its a supernet, this is whats know as CIDR.

I don't undertand what you mean with this... CIDR is a way to support classless routing. It indicates how many bits of the address are used as hosts, instead of using subnet masks. According to wikipedia (an good source of info) :

CIDR is principally a bitwise, prefix-based standard for the interpretation of IP addresses. It facilitates routing by allowing blocks of addresses to be grouped together into single routing table entries. These groups, commonly called CIDR blocks, share an initial sequence of bits in the binary representation of their IP addresses. IPv4 CIDR blocks are identified using a syntax similar to that of IPv4 addresses: a four-part dotted-decimal address, followed by a slash, then a number from 0 to 32: A.B.C.D/N. The dotted decimal portion is interpreted, like an IPv4 address, as a 32-bit binary number that has been broken into four 8-bit bytes. The number following the slash is the prefix length, the number of shared initial bits, counting from the left-hand side of the address. When speaking in abstract terms, the dotted-decimal portion is sometimes omitted, thus a /20 is a CIDR block with an unspecified 20-bit prefix

What is interesting is the comparison between CIDR and Masks:

A subnet mask is a bitmask that encodes the prefix length in a form similar to an IP address - 32 bits, starting with a number of 1 bits equal to the prefix length, ending with 0 bits, and encoded in four-part dotted-decimal format. A subnet mask encodes the same information as a prefix length, but predates the advent of CIDR.

CIDR uses variable length subnet masks (VLSM) to allocate IP addresses to subnets according to individual need, rather than some general network-wide rule. Thus the network/host division can occur at any bit boundary in the address. The process can be recursive, with a portion of the address space being further divided into even smaller portions, through the use of masks which cover more bits.

CIDR/VLSM network addresses are now used throughout the public Internet, although they are also used elsewhere, particularly in large private networks. An average desktop LAN user generally does not see them in practice, as their LAN network is usually numbered using special private RFC 1918 addresses.

You can check the URL too: http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

Hope it helps!

BubbaJ

In effect, CIDR does away with the concept of network class. An IPv4 address is really just a 32 bit number.

In classfull addressing, the division between which part of the 32 bit number is the network portion, and which is the host portion, is fixed according to certain rules.

Using CIDR, the rules regarding which part is the network, and which is the host, don't apply. You get to decide (or you are told by the network architect) how many bits are used for the network portion of the address.

Chopping the network into smaller pieces (extending the network into the host bits) is still called subnetting, but the network doesn't have to start at an 8 bit boundary, and a subnet may be what is considered a regular network under the suspended rules.

It indicates how many bits of the address are used as hosts, instead of using subnet masks. According to wikipedia (an good source of info)

Network masks are still used, and you can use the /nn notation with regular classfull addressing. Almost everything in IOS uses a network mask (exceptions like prefix lists aren't CCNA material) or wildcard (inverse of a network mask) mask. The /nn address does not specify how many bits are used for hosts, but for networks. It is only a shortcut for a full network mask, albeit a less flexible one.

Remember, anyone can contribute to Wikipedia so anything you see there should be verified. As far as the CCNA exam goes, Cisco is the authoritative source, and www.cisco.com has a search facility.

agustinchernitsky

Hi BubbaJ,

You are right, its network, not hosts... my mistake.

Saludos amigo.

eng_ahmedas

Thanks all

marlon23

CIDR is term meaned for relationships between ISPs, means Internet.

Routers in Internet (Internet, not internet) can make their routing table smaller by route summarization (like x-number of class C networks in one route) so they can route faster . Without CIDR each router should have route to every class A,B,C.. netowork, very very big table

Another advantage is that if you are ISP you can assign one subnet of class C network to one customer and another one to 2nd customer... And save your adress space and save som IPv4 adress space ass well.(without CIDR you'll have to assign whole class C network!)


You shoul read RFC 1817 (www.ietf.org/rfc/rfc1817.txt)