00000111. COMPUTERS PROCESS ONLY BINARY INFORMATION, BUT WE C...

01110011.00000111. Computers process only binary information, but we convert it to dec-imal because that is easier for us human beings to work with.

Classful Addressing

As mentioned, host addresses can belong to one of three classes of IP address, and each hasa range of addresses.The range is defined by the value of the first octet.Table 3.3 shows theclasses and their ranges, as well as the binary representations of the ranges. Classes D and Eare also classes of IP addresses, but Class D is restricted to multicasting and Class E addressesare reserved for future use. 127.0.0.0 is reserved for connectivity testing. 127.0.0.1 is a spe-cial address that represents the local loopback adapter that resolves as localhost.We can pingthe local host to troubleshoot the protocol stack.We will discuss this in more detail in the“Troubleshooting IP Addressing” section later in this chapter. Each class also has a defaultsubnet mask.

Table 3.3

IP Address Classes and Their RangesClass Range of Values Default Mask Networks Hosts BinaryA 0 to 126 255.0.0.0 126 16,777,214 00000001 to 01111110B 128 to 191 255.255.0.0 16,384 65,534 10000000 to 10111111C 192 to 223 255.255.255.0 2,097,152 254 11000000 to 11011111D 224 to 239 Not applicable Not applicable

T

EST

D

AY

T

IPIn Table 3.3, notice that the first two bits of the first octet in each class also definethe top of the range of network IDs for that class. If you take the first two bits ofClass A, 01, and add the remaining six digits as ones you get 01111111, or 127.Remember that 127 is reserved, so 126 is the highest value for the network ID of aClass A network. Class B is 10 (101111111 = 191), and Class C is 11 (11011111 =