Practical TCP/IP and Ethernet Networking- P21: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 82 Practical TCP IP and Ethernet Networking Determining the address class by inspection The NetID should normally not be all 0s as this indicates a local network. With this in mind analyze the first octet w . For class A the first bit is fixed at 0. The binary values for w can therefore only vary between 00 0 0 0 0 002 010 and 011111112 12710 . 0 is not allowed. However 127 is also a reserved number with reserved for loop-back testing. In particular is used to test that the TCP IP protocol is properly configured by sending information in a loop back to the computer that originally sent the packet without it traveling over the network. The values for w can therefore only vary between 1 and 126 which allows for 126 possible class A NetIDs. For class B the first two bits are fixed at 10. The binary values for w can therefore only vary between 100000002 12810 and 101111112 19110 . For class C the first three bits are fixed at 110. The binary values for w can therefore only vary between 110000002 19210 and 110111112 22310 . The relationship between w and the address class can therefore be summarized as follows. Figure IPv4 address range vs class Number of networks and hosts per address class Note that there are two reserved host numbers irrespective of class. These are all zeros or all ones for HostID. An IP address with a host number of zero is used as the address of the whole network. For example on a class C network with the NetID the IP address indicates the whole network. If all the bits of the HostID are set to 1 for example then a broadcast message will be sent to every host on that network. To summarize HostID all zeros means this network. HostID all ones means all hosts on this network For class A the number of NetIDs is determined by octet w . Unfortunately the first bit fixed at 0 is used to indicate class A and hence cannot be used. This leaves seven usable bits. Seven bits allow .
