计算机网络课件(谢希仁) 网络层.pptVIP

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * The IP address is 32 bits in length and has two parts: Network number Host number The address format is known as dotted-decimal notation. Example address: 04. Each bit in the octet has a binary weight, such as (128,...4, 2, 1). The minimum value for an octet is 0; it contains all zeros. The maximum value for an octet is 255; it contains all ones. The allocation of addresses is managed by a central authority. * When IP was first developed, there were no classes of addresses. Now, for ease of administration, the IP addresses are broken up into classes. There are only 126 Class A address spaces, but each one can contain approximately 16million hosts. There are 65,534 Class B address spaces with 65,534 hosts each. There are more than 16 million Class C address spaces possible, but they only have 254 hosts each. This scheme allows the administrative authority to assign addresses based on the size of the network. That authority designed this system on the assumption that there would be many more small networks than large networks in the world. Note: Class D and E addresses are also defined. Class D addresses start at and are used for multicast purposes. Class E addresses start at and are used for experimental purposes. * The most significant bit pattern determines the class of the address, as well as how many bits make up the network portion of the address. Class A addresses include Range of network numbers: to Number of host addresses: 16,777,214 Class B addresses include Range of network numbers: to Number of host addresses: 65,534 Class C addresses include Range of network numbers: to Number of host addresses: 254 Class D addresses include Range of network numbers: to 54 * The first octet rule states that the class of an address can be determined by the numerical value of the first octet. Once the first octet rule is applied, the router identifies how many bits it must match to