Binary and Dotted Decimal NotationSo far, all of the IP addresses in this section have been represented in dotted decimal notation. Dotted decimal notation, the most common way of expressing IP addresses, refers tothe “shorthand”convention used to represent IP addresses and make them easy for peopleto read. In dotted decimal notation, a decimal number between 0 and 255 represents eachbinary octet, for a total of 256 possibilities. A period, or dot, separates each decimal. Anexample of a dotted decimal IP address is 131.65.10.18.Each number in a dotted decimal address has a binary equivalent. In Chapter 3, you learnedhow to convert decimal numbers to their binary equivalents. Converting a dotted decimaladdress to its binary equivalent is simply a matter of converting each octet and removing thedecimal points. For example, in the dotted decimal address 131.65.10.36, the binary equivalent of the first octet, 131, is 10000011; the binary equivalent of the second octet, 65, is01000001; the binary equivalent of the third octet, 10, is 00001010; and the binary equivalent of the fourth octet, 36, is 00100100. Therefore, the binary value for 131.65.10.36 is10000011 01000001 00001010 00100100.Subnet MaskIn addition to an IP address, every device on a network running IPv4 is assigned a subnet mask. Asubnet maskis a special 32-bit number that, when combined with a device’sIP address, informs the rest of the network about the segment or network to which thedevice is attached. That is, it identifies the device’s subnet. Like IP addresses, subnet
masks are composed of four octets (32 bits) and can be expressed in either binary or
dotted decimal notation. Subnet masks are assigned in the same way that IP addresses
are assigned—either manually, within a device’s TCP/IP configuration, or automatically,
through a service such as DHCP (described in detail later in this chapter). A more common term for subnet mask isnet mask, and sometimes simply mask,asin“adevice’s
mask.”
You might wonder why a network node even needs a subnet mask, given that the first
octet of its IP address indicates its network class. The answer lies withsubnetting,aprocess of subdividing a single class of networks into multiple, smaller logical networks, or
segments.
Network managers create subnets to manage and separate network traffic and to make the
best use of a limited number of IP addresses. Methods of subnetting are discussed in detail
in Chapter 9. For now, it is enough to know that regardless of whether a network is subnetted, its devices are assigned a subnet mask.
On networks that use subnetting, the subnet mask varies depending on the way the
network is subnetted. On networks that do not use subnetting, however, the subnet
masks take on a default value, as shown in Table 4-5. To qualify for Network+ certification, you should be familiar with the default subnet masks associated with each
network class
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