CertiGuide to A+ (A+ 4 Real)  9  Chapter 14: Networks       9  What is a Network?            9  Protocol Standards

Classless Inter-Domain Routing (CIDR)

The main disadvantage of this type of classful subnetting is that it can waste a large amount of IP addresses. If you require more than 254 IP addresses you would need to use a class B subnet, which contains 65533 addresses – far more than usually required. To solve this, CIDR is used. CIDR, standing for Classless Inter Domain Routing, allows for far more precise allocation of addresses.

The easiest way to explain CIDR is by reviewing some binary! Whereas a classful subnet is expressed using decimal notation (i.e.: 255.255.255.0), CIDR subnetting is expressed using the number of bits in the network portion of an IP address:

• Classful: 255.255.255.0

• Binary: 11111111.11111111.11111111.000

• CIDR: /24

How have all those 1’s and the 24 been calculated? Quite simply, convert the decimal subnet mask into binary. A subnet octet can run from 0 to 255, providing 256 individual values. In binary, 256 is 11111111 – 8 individual bits. Therefore we can easily convert the classful subnet to a binary representation. The final step is to count the number of “on” bits in the binary representation. In the above example, there are 24 “on” bits (each signified by a “1”), therefore the CIDR address is expressed as “192.168.1.1/24”.

If we wanted to narrow the network range further, we could use a subnet that provided only 128 addresses (remember that the first and last addresses must not be used, so only 126 are available for use).

• IP address range: 192.168.1.0 to 192.168.1.127

• Binary: 11111111.11111111.11111111.10000000

• CIDR: /25

In this example, we have added an extra binary “on” to the last 8 bits. 100000000 in binary is equal to 128 in decimal, therefore we are specifying that this network has 128 addresses in it. And because we now have 25 “on” bits in the binary mask, the CIDR subnet is now /25. In IP address terms, this means that 192.168.1.1 and 192.168.1.126 are in the same network, which is a different network to 192.168.1.129 and 192.168.1.254. With a classful class C subnet, all 4 addresses would be in the same network.

A handy tool to assist with subnet and network calculations is WildPackets IP Subnet Calculator, which is a free download at: http://www.wildpackets.com/products/ipsubnetcalculator

Returning to our example Windows 2000 network configuration. In the Network Connection dialog, select “Internet Protocol (TCP/IP)”, and click Properties. The dialog in Figure 435 appears.

This dialog allows you to configure basic IP properties of your connection. The top half screenshot shows a static address assignment – one that has been manually configured on the client, and cannot be automatically changed (or reclaimed) via DHCP. As discussed, in the screenshot an RFC 1918 private IP address has been configured – if we want this machine to access the Internet, the default gateway at 192.168.0.200 had better be running NAT! The bottom half of the screenshot is the part that allows a network to be human-friendly, discussed next.

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