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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing
                          9  IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting"

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IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting"
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IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation
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IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages
(Page 3 of 3)

The Many Benefits of Classless Addressing and Routing

CIDR provides numerous advantages over the “classful” addressing scheme, whether or not subnetting is used:

  • Efficient Address Space Allocation: Instead of allocating addresses in fixed-size blocks of low granularity, under CIDR addresses are allocated in sizes of any binary multiple. So, a company that needs 5,000 addresses can be assigned a block of 8,190 instead of 65,534, as shown in Figure 81. Or, to think of it another way, the equivalent of a single Class B network can be shared amongst 8 companies that each need 8,190 or fewer IP addresses.

  • Elimination of Class Imbalances: There are no more class A, B and C networks, so there is no problem with some portions of the address space being widely used while others are neglected.

  • Efficient Routing Entries: CIDR's multiple-level hierarchical structure allows a small number of routing entries to represent a large number of networks. Network descriptions can be “aggregated” and represented by a single entry. Since CIDR is hierarchical, the detail of lower-level, smaller networks can be hidden from routers that move traffic between large groups of networks. This is discussed more completely in the section on IP routing issues.

  • No Separate Subnetting Method: CIDR implements the concepts of subnetting within the internet itself. An organization can use the same method used on the Internet to subdivide its internal network into subnets of arbitrary complexity without needing a separate subnetting mechanism.

    Figure 81: Classless Addressing (CIDR) Solves The “Granularity Problem”

    Figure 64 illustrated the primary problem with “classful” addressing: the great distance between the size of Class B and Class C networks. CIDR solves this issue by allowing any number of bits to be used for the network ID. In the case of an organization with 5,000 hosts, a /19 network with 8,190 hosts can be assigned. This reduces the address space waste for such an organization by about 95%.


The Main Disadvantage of CIDR: Complexity

Since the main benefit of “classful” addressing was its simplicity, it's no surprise that the main drawback of CIDR is its greater complexity. One issue is that it is no longer possible to determine by looking at the first octet to determine how many bits of an IP address represent the network ID and how many the host ID. A bit more care needs to be used in setting up routers as well, to make sure that routing is accomplished correctly.

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Version 3.0 - Version Date: September 20, 2005

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