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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 and "Supernetting" Overview, Motivation, Advantages and Disadvantages
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34
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IP Classless Addressing Block Sizes and "Classful" Network Equivalents
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IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation
(Page 2 of 4)

CIDR ("Slash") Notation

Just as subnetting required the use of a subnet mask to show which bits belong to the network ID or subnet ID and which to the host ID, CIDR uses a subnet mask to show where the line is drawn between host ID and network ID. However, for simplicity, under CIDR we don't usually work with 32-bit binary subnet masks. Instead, we use slash notation, more properly called CIDR notation. In this method, we show the size of the network, sometimes called the prefix length, by following an IP address by an integer that tells us how many bits are used for the network ID (prefix).

Key Concept: Since there are no address classes in CIDR, one cannot tell the size of the network ID of an address from the address alone. In CIDR, the length of the prefix (network ID) is indicated by placing it following a slash after the address. This is called CIDR notation or slash notation.


For example, consider the network specification 184.13.152.0/22. The “22” means this network has 22 bits for the network ID and 10 bits for the host ID. This is equivalent to specifying a network with an address of 184.13.152.0 and a subnet mask of 255.255.252.0, as you can see in Figure 82. This sample network provides a total of 1,022 hosts (210 minus 2). The table in the following topic shows all the different possible network sizes that can be configured under CIDR.


Figure 82: CIDR (“Slash”) Notation and Its Subnet Mask Equivalent

A classless network is normally specified in CIDR or “slash” notation, such as this example: 184.13.152.0/22. Here, the “/22” means the first 22 bits of the address are the network ID. The equivalent subnet mask can be calculated by creating a 32-bit number with 22 ones followed by 10 zeroes.

 


Note: You may recall my mentioning that under “classful” subnetting, the bits used for the subnet ID did not need to be contiguous. Even though to avoid confusion this was rarely if ever used, non-contiguous subnet ID bits were possible. Under CIDR, the requirement for contiguous subnet ID bits has been made official—you could not use slash notation otherwise.



Previous Topic/Section
IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages
Previous Page
Pages in Current Topic/Section
1
2
34
Next Page
IP Classless Addressing Block Sizes and "Classful" Network Equivalents
Next Topic/Section

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