PDF《ISI》

4 forms or classes. The first type of address, or class A, has a 7-bit network number and a 24-bit local address. The highest-order bit is set to 0. This allows

128 class A networks.

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3 0

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1 |0| NETWORK | Local Address | Class A Address The second type of address, class B, has a 14-bit network number and a 16-bit local address. The two highest-order bits are set to 1-0. This allows 16,384 class B networks.

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1 |1 0| NETWORK | Local Address | Class B Address The third type of address, class C, has a 21-bit network number and a 8-bit local address. The three highest-order bits are set to 1-1-0. This allows 2,097,152 class C networks.

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1 0| NETWORK | Local Address | Class C Address Reynolds &

Postel [Page 3] Internet Numbers RFC

997 Network Numbers The fourth type of address, class D, is used as a multicast address [10]. The four highest-order bits are set to 1-1-1-0.

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1 1 0| multicast address | Class D Address Note: No addresses are allowed with the four highest-order bits set to 1-1-1-1. These addresses, called class E , are reserved. One commonly used notation for internet host addresses divides the 32-bit address into four 8-bit fields and specifies the value of each field as a decimal number with the fields separated by periods. This is called the dotted decimal notation. For example, the internet address of VENERA.ISI.EDU in dotted decimal is 010.001.000.052, or 10.1.0.52. The dotted decimal notation will be used in the listing of assigned network numbers. The class A networks will have nnn.rrr.rrr.rrr, the class B networks will have nnn.nnn.rrr.rrr, and the class C networks will have nnn.nnn.nnn.rrr, where nnn represents part or all of a network number and rrr represents part or all of a local address. There are four catagories of users of Internet Addresses: Research, Defense, Government (Non-Defense), and Commercial. To reflect the allocation of network identifiers among the categories, a one-character code is placed to the left of the network number: R for Research, D for Defense, G for Government, and C for Commercial (see Appendix A for further details on this division of the network identification). Network numbers are assigned for networks that are connected to the research Internet and operational Internet, and for independent networks that use the IP family protocols (these are usually commercial). These independent networks are marked with an asterisk preceding the number. The administrators of independent networks must apply separately for permission to interconnect their network with the Internet. Independent networks should not be listed in the working tables of the Internet hosts or gateways. For various reasons, the assigned numbers of networks are sometimes changed. To ease the transition the old number will be listed for a Reynolds &

Postel [Page 4] Internet Numbers RFC

997 Network Numbers transition period as well. These old number entries will be marked with a T following the number and preceding the name, and the network name will be suffixed -TEMP . Special Addresses: In certain contexts, it is useful to have fixed addresses with functional significance rather than as identifiers of specific hosts. The address zero is to be interpreted as meaning this , as in this network . For example, the address 0.0.0.37 could be interpreted as meaning host