通信外文翻譯---了解 tcpip 尋址和子網(wǎng)配置基礎(chǔ)知識(shí)

1、<p>　　Understanding TCPIP addressing and subnetting basics</p><p>　　INTRODUCTION</p><p>　　When you configure the TCP/IP protocol on a Microsoft Windows computer, an IP address, subnet mask,

2、 and usually a default gateway are required in the TCP/IP configuration settings.</p><p>　　To configure TCP/IP correctly, it is necessary to understand how TCP/IP networks are addressed and divided into netw

3、orks and subnetworks. This article is intended as a general introduction to the concepts of IP networks and subnetting. A glossary is included at the end of article.</p><p>　　Back to the top</p><p

4、>　　MORE INFORMATION</p><p>　　The success of TCP/IP as the network protocol of the Internet is largely because of its ability to connect together networks of different sizes and systems of different types.

5、 These networks are arbitrarily defined into three main classes (along with a few others) that have predefined sizes, each of which can be divided into smaller subnetworks by system administrators. A subnet mask is used

6、to divide an IP address into two parts. One part identifies the host (computer), the other part identifie</p><p>　　Back to the top</p><p>　　IP addresses: Networks and hosts</p><p>　

7、　An IP address is a 32-bit number that uniquely identifies a host (computer or other device, such as a printer or router) on a TCP/IP network.</p><p>　　IP addresses are normally expressed in dotted-decimal f

8、ormat, with four numbers separated by periods, such as 192.168.123.132. To understand how subnet masks are used to distinguish between hosts, networks, and subnetworks, examine an IP address in binary notation.</p>

9、<p>　　For example, the dotted-decimal IP address 192.168.123.132 is (in binary notation) the 32 bit number 110000000101000111101110000100. This number may be hard to make sense of, so divide it into four parts of

10、eight binary digits.</p><p>　　These eight bit sections are known as octets. The example IP address, then, becomes 11000000.10101000.01111011.10000100. This number only makes a little more sense, so for most

11、uses, convert the binary address into dotted-decimal format (192.168.123.132). The decimal numbers separated by periods are the octets converted from binary to decimal notation.</p><p>　　For a TCP/IP wide ar

12、ea network (WAN) to work efficiently as a collection of networks, the routers that pass packets of data between networks do not know the exact location of a host for which a packet of information is destined. Routers onl

13、y know what network the host is a member of and use information stored in their route table to determine how to get the packet to the destination host's network. After the packet is delivered to the destination's

14、 network, the packet is delivered to the appropr</p><p>　　For this process to work, an IP address has two parts. The first part of an IP address is used as a network address, the last part as a host address.

15、 If you take the example 192.168.123.132 and divide it into these two parts you get the following:</p><p>　　192.168.123. Network</p><p><b>　　.132 Host</b></p><p><

16、b>　　-or-</b></p><p>　　192.168.123.0 - network address.</p><p>　　0.0.0.132 - host address.</p><p>　　Back to the top</p><p>　　Subnet mask</p><p>

17、;　　The second item, which is required for TCP/IP to work, is the subnet mask. The subnet mask is used by the TCP/IP protocol to determine whether a host is on the local subnet or on a remote network.</p><p>

18、　　In TCP/IP, the parts of the IP address that are used as the network and host addresses are not fixed, so the network and host addresses above cannot be determined unless you have more information. This information is s

19、upplied in another 32-bit number called a subnet mask. In this example, the subnet mask is 255.255.255.0. It is not obvious what this number means unless you know that 255 in binary notation equals 11111111; so, the subn

20、et mask is:</p><p>　　11111111.11111111.11111111.0000000</p><p>　　Lining up the IP address and the subnet mask together, the network and host portions of the address can be separated:</p>

21、<p>　　11000000.10101000.01111011.10000100 -- IP address (192.168.123.132)</p><p>　　11111111.11111111.11111111.00000000 -- Subnet mask (255.255.255.0)</p><p>　　The first 24 bits (the number

22、of ones in the subnet mask) are identified as the network address, with the last 8 bits (the number of remaining zeros in the subnet mask) identified as the host address. This gives you the following:</p><p>

23、;　　11000000.10101000.01111011.00000000 -- Network address (192.168.123.0)</p><p>　　00000000.00000000.00000000.10000100 -- Host address (000.000.000.132)</p><p>　　So now you know, for this exampl

24、e using a 255.255.255.0 subnet mask, that the network ID is 192.168.123.0, and the host address is 0.0.0.132. When a packet arrives on the 192.168.123.0 subnet (from the local subnet or a remote network), and it has a de

25、stination address of 192.168.123.132, your computer will receive it from the network and process it.</p><p>　　Almost all decimal subnet masks convert to binary numbers that are all ones on the left and all z

26、eros on the right. Some other common subnet masks are:</p><p>　　Decimal Binary</p><p>　　255.255.255.192 1111111.11111111.1111111.11000000</p><p>　　255.255.25

27、5.224 1111111.11111111.1111111.11100000</p><p>　　Internet RFC 1878 (available from http://www.internic.net ) describes the valid subnets and subnet masks that can be used on TCP/IP networks.</p>

28、;<p>　　Back to the top</p><p>　　Network classes</p><p>　　Internet addresses are allocated by the InterNIC (http://www.internic.net ), the organization that administers the Internet. These

29、 IP addresses are divided into classes. The most common of these are classes A, B, and C. Classes D and E exist, but are not generally used by end users. Each of the address classes has a different default subnet mask. Y

30、ou can identify the class of an IP address by looking at its first octet. Following are the ranges of Class A, B, and C Internet addresses, each with </p><p>　　Class A networks use a default subnet mask of 2

31、55.0.0.0 and have 0-127 as their first octet. The address 10.52.36.11 is a class A address. Its first octet is 10, which is between 1 and 126, inclusive.</p><p>　　Class B networks use a default subnet mask o

32、f 255.255.0.0 and have 128-191 as their first octet. The address 172.16.52.63 is a class B address. Its first octet is 172, which is between 128 and 191, inclusive.</p><p>　　Class C networks use a default su

33、bnet mask of 255.255.255.0 and have 192-223 as their first octet. The address 192.168.123.132 is a class C address. Its first octet is 192, which is between 192 and 223, inclusive.</p><p>　　In some scenarios

34、, the default subnet mask values do not fit the needs of the organization, because of the physical topology of the network, or because the numbers of networks (or hosts) do not fit within the default subnet mask restrict

35、ions. The next section explains how networks can be divided using subnet masks.</p><p>　　Back to the top</p><p>　　Subnetting</p><p>　　A Class A, B, or C TCP/IP network can be furthe

36、r divided, or subnetted, by a system administrator. This becomes necessary as you reconcile the logical address scheme of the Internet (the abstract world of IP addresses and subnets) with the physical networks in use by

37、 the real world.</p><p>　　A system administrator who is allocated a block of IP addresses may be administering networks that are not organized in a way that easily fits these addresses. For example, you have

38、 a wide area network with 150 hosts on three networks (in different cities) that are connected by a TCP/IP router. Each of these three networks has 50 hosts. You are allocated the class C network 192.168.123.0. (For illu

39、stration, this address is actually from a range that is not allocated on the Internet.) This means </p><p>　　Two addresses that cannot be used in your example are 192.168.123.0 and 192.168.123.255 because bi

40、nary addresses with a host portion of all ones and all zeros are invalid. The zero address is invalid because it is used to specify a network without specifying a host. The 255 address (in binary notation, a host address

41、 of all ones) is used to broadcast a message to every host on a network. Just remember that the first and last address in any network or subnet cannot be assigned to any individual h</p><p>　　You should now

42、be able to give IP addresses to 254 hosts. This works fine if all 150 computers are on a single network. However, your 150 computers are on three separate physical networks. Instead of requesting more address blocks for

43、each network, you divide your network into subnets that enable you to use one block of addresses on multiple physical networks.</p><p>　　In this case, you divide your network into four subnets by using a sub

44、net mask that makes the network address larger and the possible range of host addresses smaller. In other words, you are 'borrowing' some of the bits usually used for the host address, and using them for the netw

45、ork portion of the address. The subnet mask 255.255.255.192 gives you four networks of 62 hosts each. This works because in binary notation, 255.255.255.192 is the same as 1111111.11111111.1111111.11000000. The first t&l

46、t;/p><p>　　Using a subnet mask of 255.255.255.192, your 192.168.123.0 network then becomes the four networks 192.168.123.0, 192.168.123.64, 192.168.123.128 and 192.168.123.192. These four networks would have as

47、 valid host addresses:</p><p>　　192.168.123.1-62</p><p>　　192.168.123.65-126</p><p>　　192.168.123.129-190</p><p>　　192.168.123.193-254</p><p>　　Remember, a

48、gain, that binary host addresses with all ones or all zeros are invalid, so you cannot use addresses with the last octet of 0, 63, 64, 127, 128, 191, 192, or 255.</p><p>　　You can see how this works by looki

49、ng at two host addresses, 192.168.123.71 and 192.168.123.133. If you used the default Class C subnet mask of 255.255.255.0, both addresses are on the 192.168.123.0 network. However, if you use the subnet mask of 255.255.

50、255.192, they are on different networks; 192.168.123.71 is on the 192.168.123.64 network, 192.168.123.133 is on the 192.168.123.128 network.</p><p>　　Back to the top</p><p>　　Default gateways<

51、;/p><p>　　If a TCP/IP computer needs to communicate with a host on another network, it will usually communicate through a device called a router. In TCP/IP terms, a router that is specified on a host, which lin

52、ks the host's subnet to other networks, is called a default gateway. This section explains how TCP/IP determines whether or not to send packets to its default gateway to reach another computer or device on the networ

53、k.</p><p>　　When a host attempts to communicate with another device using TCP/IP, it performs a comparison process using the defined subnet mask and the destination IP address versus the subnet mask and its

54、own IP address. The result of this comparison tells the computer whether the destination is a local host or a remote host.</p><p>　　If the result of this process determines the destination to be a local host

55、, then the computer will simply send the packet on the local subnet. If the result of the comparison determines the destination to be a remote host, then the computer will forward the packet to the default gateway define

56、d in its TCP/IP properties. It is then the responsibility of the router to forward the packet to the correct subnet.</p><p>　　Back to the top</p><p>　　Troubleshooting</p><p>　　TCP/I

57、P network problems are often caused by incorrect configuration of the three main entries in a computer's TCP/IP properties. By understanding how errors in TCP/IP configuration affect network operations, you can solve

58、 many common TCP/IP problems.</p><p>　　Incorrect Subnet Mask: If a network uses a subnet mask other than the default mask for its address class, and a client is still configured with the default subnet mask

59、for the address class, communication will fail to some nearby networks but not to distant ones. As an example, if you create four subnets (such as in the subnetting example) but use the incorrect subnet mask of 255.255.2

60、55.0 in your TCP/IP configuration, hosts will not be able to determine that some computers are on different subn</p><p>　　Incorrect IP Address: If you put computers with IP addresses that should be on separa

61、te subnets on a local network with each other, they will not be able to communicate. They will try to send packets to each other through a router that will not be able to forward them correctly. A symptom of this problem

62、 is a computer that can talk to hosts on remote networks, but cannot communicate with some or all computers on their local network. To correct this problem, make sure all computers on the same ph</p><p>　　In

63、correct Default Gateway: A computer configured with an incorrect default gateway will be able to communicate with hosts on its own network segment, but will fail to communicate with hosts on some or all remote networks.

64、If a single physical network has more than one router, and the wrong router is configured as a default gateway, a host will be able to communicate with some remote networks, but not others. This problem is common if an o

65、rganization has a router to an internal TCP/IP network and</p><p>　　REFERENCES</p><p>　　Two popular references on TCP/IP are:</p><p>　　"TCP/IP Illustrated, Volume 1: The Protoc

66、ols," Richard Stevens, Addison Wesley, 1994</p><p>　　"Internetworking with TCP/IP, Volume 1: Principles, Protocols, and Architecture," Douglas E. Comer, Prentice Hall, 1995</p><p>

67、;　　It is strongly recommended that a system administrator responsible for TCP/IP networks have at least one of these references available.</p><p><b>　　Glossary</b></p><p>　　Broadcast

68、 address -- An IP address with a host portion that is all ones.</p><p>　　Host -- A computer or other device on a TCP/IP network.</p><p>　　Internet -- The global collection of networks that are c

69、onnected together and share a common range of IP addresses.</p><p>　　InterNIC -- The organization responsible for administration of IP addresses on the Internet.</p><p>　　IP -- The network proto

70、col used for sending network packets over a TCP/IP network or the Internet.</p><p>　　IP Address -- A unique 32-bit address for a host on a TCP/IP network or internetwork.</p><p>　　Network -- The

71、re are two uses of the term network in this article. One is a group of computers on a single physical network segment; the other is an IP network address range that is allocated by a system administrator.</p><

72、p>　　Network address -- An IP address with a host portion that is all zeros.</p><p>　　Octet -- An 8-bit number, 4 of which comprise a 32-bit IP address. They have a range of 00000000-11111111 that correspo

73、nd to the decimal values 0- 255.</p><p>　　Packet -- A unit of data passed over a TCP/IP network or wide area network.</p><p>　　RFC (Request for Comment) -- A document used to define standards on

74、 the Internet.</p><p>　　Router -- A device that passes network traffic between different IP networks.</p><p>　　Subnet Mask -- A 32-bit number used to distinguish the network and host portions of

75、 an IP address.</p><p>　　Subnet or Subnetwork -- A smaller network created by dividing a larger network into equal parts.</p><p>　　TCP/IP -- Used broadly, the set of protocols, standards and uti

76、lities commonly used on the Internet and large networks.</p><p>　　Wide area network (WAN) -- A large network that is a collection of smaller networks separated by routers. The Internet is an example of a ver

77、y large WAN.</p><p>　　了解 TCPIP 尋址和子網(wǎng)配置基礎(chǔ)知識(shí)</p><p><b>　　簡(jiǎn)介</b></p><p>　　在 Microsoft Windows 計(jì)算機(jī)上配置 TCP/IP 協(xié)議時(shí)，TCP/IP 配置設(shè)置中需要 IP 地址和子網(wǎng)掩碼，通常還需要一個(gè)默認(rèn)網(wǎng)關(guān)。</p><p>　

78、　要正確配置 TCP/IP，有必要了解 TCP/IP 網(wǎng)絡(luò)的尋址方式以及網(wǎng)絡(luò)和子網(wǎng)的劃分方式。本文旨在對(duì) IP 網(wǎng)絡(luò)和子網(wǎng)的概念進(jìn)行一般性介紹。本文結(jié)尾包含一個(gè)術(shù)語(yǔ)表。</p><p><b>　　更多信息</b></p><p>　　作為 Internet 的網(wǎng)絡(luò)協(xié)議，TCP/IP 的成功在很大程度上歸功于它將不同大小的網(wǎng)絡(luò)和不同類型的系統(tǒng)連接在一起的能力。這些網(wǎng)絡(luò)

79、被強(qiáng)制定義為具有預(yù)定義大小的三個(gè)主要類（還有其他一些類別），每一類都可以由系統(tǒng)管理員分成更小的子網(wǎng)。子網(wǎng)掩碼用于將 IP 地址分成兩個(gè)部分。一部分標(biāo)識(shí)主機(jī)（計(jì)算機(jī)），另一部分標(biāo)識(shí)它所屬的網(wǎng)絡(luò)。查看 IP（Internet 協(xié)議）地址并研究它的組織方式可以幫助您更好地理解 IP 地址和子網(wǎng)掩碼的工作方式。</p><p>　　IP 地址：網(wǎng)絡(luò)和主機(jī)</p><p>　　IP 地址是一個(gè) 32

80、 位數(shù)字，它唯一地標(biāo)識(shí) TCP/IP 網(wǎng)絡(luò)上的主機(jī)（計(jì)算機(jī)或其他設(shè)備，如打印機(jī)或路由器）。</p><p>　　IP 地址通常以點(diǎn)分十進(jìn)制格式表示，四個(gè)數(shù)字由句點(diǎn)分隔，例如 192.168.123.132。要了解子網(wǎng)掩碼如何用于區(qū)分不同的主機(jī)、網(wǎng)絡(luò)和子網(wǎng)，請(qǐng)查看以二進(jìn)制表示的 IP 地址。</p><p>　　例如，點(diǎn)分十進(jìn)制 IP 地址 192.168.123.132 在二進(jìn)制表示法中為

81、 32 位數(shù)字 110000000101000111101110000100。此數(shù)字的含義可能很難理解，因此它被分成四個(gè)部分，每個(gè)部分有八個(gè)二進(jìn)制數(shù)字。</p><p>　　這些八位部分稱為八位組。示例 IP 地址于是變成 11000000.10101000.01111011.10000100。此數(shù)字表示的含義只是稍微明白了一點(diǎn)，因此在大多數(shù)應(yīng)用中，會(huì)將二進(jìn)制地址轉(zhuǎn)換為點(diǎn)分十進(jìn)制格式 (192.168.123.1

82、32)。點(diǎn)分十進(jìn)制數(shù)字是從二進(jìn)制轉(zhuǎn)換為十進(jìn)制表示法的八位組。</p><p>　　為了讓 TCP/IP 廣域網(wǎng) (WAN) 以一個(gè)網(wǎng)絡(luò)集的形式高效工作，在網(wǎng)絡(luò)間傳遞數(shù)據(jù)包的路由器并不知道信息包的目標(biāo)主機(jī)的確切位置。路由器只知道主機(jī)是哪一個(gè)網(wǎng)絡(luò)的成員，并使用存儲(chǔ)在路由表中的信息來(lái)確定如何將數(shù)據(jù)包送達(dá)目標(biāo)主機(jī)的網(wǎng)絡(luò)。當(dāng)數(shù)據(jù)包被傳送到目標(biāo)網(wǎng)絡(luò)后，該數(shù)據(jù)包就會(huì)被傳送到相應(yīng)的主機(jī)。</p><p>

83、　　為了讓此過(guò)程順利進(jìn)行，IP 地址分為兩個(gè)部分。IP 地址的前一部分作為網(wǎng)絡(luò)地址，后一部分作為主機(jī)地址。以 192.168.123.132 為例，將它分為這兩個(gè)部分之后，會(huì)得到：</p><p>　　192.168.123. 網(wǎng)絡(luò)</p><p><b>　　.132 主機(jī)</b></p><p><b>　　- 或 -&

84、lt;/b></p><p>　　192.168.123.0 — 網(wǎng)絡(luò)地址。</p><p>　　0.0.0.132 — 主機(jī)地址。</p><p><b>　　子網(wǎng)掩碼</b></p><p>　　第二項(xiàng)是子網(wǎng)掩碼，它是 TCP/IP 正常工作所必需的。TCP/IP 協(xié)議使用子網(wǎng)掩碼確定主機(jī)是在本地子網(wǎng)中

85、還是在遠(yuǎn)程網(wǎng)絡(luò)中。</p><p>　　在 TCP/IP 中，將哪部分 IP 地址用作網(wǎng)絡(luò)地址和主機(jī)地址并不固定，所以除非您掌握詳細(xì)的信息，否則無(wú)法確定上述網(wǎng)絡(luò)地址和主機(jī)地址。此信息在另一個(gè) 32 位數(shù)字中提供，稱為子網(wǎng)掩碼。在本例中，子網(wǎng)掩碼為 255.255.255.0。如果您不知道二進(jìn)制表示法中的 255 等于 11111111，可能并不清楚該數(shù)字表示的含義；照此分析，子網(wǎng)掩碼為：</p>&

86、lt;p>　　11111111.11111111.11111111.0000000</p><p>　　將 IP 地址和子網(wǎng)掩碼排列在一起比較，就可以分清該地址的網(wǎng)絡(luò)部分和主機(jī)部分：</p><p>　　11000000.10101000.01111011.10000100 -- IP 地址 (192.168.123.132)</p><p>　　111

87、11111.11111111.11111111.00000000 -- 子網(wǎng)掩碼 (255.255.255.0)</p><p>　　前 24 位（子網(wǎng)掩碼中的數(shù)字 1）被標(biāo)識(shí)為網(wǎng)絡(luò)地址，后 8 位（子網(wǎng)掩碼中剩余的數(shù)字 0）被標(biāo)識(shí)為主機(jī)地址。據(jù)此可以得到：</p><p>　　11000000.10101000.01111011.00000000 -- 網(wǎng)絡(luò)地址 (192.168.123

88、.0)</p><p>　　00000000.00000000.00000000.10000100 -- 主機(jī)地址 (000.000.000.132)</p><p>　　這樣，我們就可以知道，在這個(gè)使用 255.255.255.0 子網(wǎng)掩碼的示例中，網(wǎng)絡(luò) ID 為 192.168.123.0，主機(jī)地址為 0.0.0.132。當(dāng)數(shù)據(jù)包到達(dá) 192.168.123.0 子網(wǎng)（從本地子網(wǎng)或遠(yuǎn)程

89、網(wǎng)絡(luò)），而且它的目標(biāo)地址為 192.168.123.132 時(shí)，您的計(jì)算機(jī)將從網(wǎng)絡(luò)接收它并對(duì)它進(jìn)行處理。</p><p>　　幾乎所有十進(jìn)制子網(wǎng)掩碼都轉(zhuǎn)換為左側(cè)全部是一、右側(cè)全部是零的二進(jìn)制數(shù)字。其他一些常見的子網(wǎng)掩碼有：</p><p>　　十進(jìn)制 二進(jìn)制</p><p>　　255.255.255.192 111

90、1111.11111111.1111111.11000000</p><p>　　255.255.255.224 1111111.11111111.1111111.11100000</p><p>　　Internet RFC 1878（從http://www.internic.net獲?。┟枋隽丝稍?TCP/IP 網(wǎng)絡(luò)中使用的有效子網(wǎng)和子網(wǎng)掩碼。</p>&

91、lt;p><b>　　網(wǎng)絡(luò)類</b></p><p>　　Internet 地址由管理 Internet 的機(jī)構(gòu) InterNIC (http://www.internic.net) 來(lái)分配。這些 IP 地址分成若干類。其中最常見的是 A、B 和 C 類。也有 D 和 E 類，但是最終用戶通常不會(huì)使用。每個(gè)地址類都有不同的默認(rèn)子網(wǎng)掩碼?？梢酝ㄟ^(guò)查看 IP 地址的第一個(gè)八位組來(lái)識(shí)別該 I

92、P 地址的類別。下面是 A、B 和 C 類 Internet 地址的范圍，每一類地址都有一個(gè)示例：</p><p>　　A 類網(wǎng)絡(luò)使用的默認(rèn)子網(wǎng)掩碼為 255.0.0.0，第一個(gè)八位組為 0-127。地址 10.52.36.11 就是一個(gè) A 類地址。它的第一個(gè)八位組為 10，介于 1 至 126 之間（包括 1 和 126）。</p><p>　　B 類網(wǎng)絡(luò)使用的默認(rèn)子網(wǎng)掩碼為 255.

93、255.0.0，第一個(gè)八位組為 128-191。地址 172.16.52.63 就是一個(gè) B 類地址。它的第一個(gè)八位組為 172，介于 128 至 191 之間（包括 128 和 191）。</p><p>　　C 類網(wǎng)絡(luò)使用的默認(rèn)子網(wǎng)掩碼為 255.255.255.0，第一個(gè)八位組為 192-223。地址 192.168.123.132 就是一個(gè) C 類地址。它的第一個(gè)八位組為 192，介于 192 至 223

94、 之間（包括 192 和 223）。</p><p>　　在某些情況下，由于網(wǎng)絡(luò)的物理拓?fù)浠蛞驗(yàn)榫W(wǎng)絡(luò)（或主機(jī)）的數(shù)目在默認(rèn)的子網(wǎng)掩碼限制之下并不適用，所以默認(rèn)子網(wǎng)掩碼值可能不適合機(jī)構(gòu)的需要。下一部分將解釋如何使用子網(wǎng)掩碼劃分網(wǎng)絡(luò)。</p><p><b>　　子網(wǎng)配置</b></p><p>　　系統(tǒng)管理員可以進(jìn)一步劃分 A、B 或 C 類 T

95、CP/IP 網(wǎng)絡(luò)或?qū)@些網(wǎng)絡(luò)進(jìn)行子網(wǎng)配置。當(dāng)您將 Internet 的邏輯地址結(jié)構(gòu)（IP 地址和子網(wǎng)的抽象世界）與真實(shí)世界中使用的物理網(wǎng)絡(luò)進(jìn)行協(xié)調(diào)時(shí)，就有必要進(jìn)行子網(wǎng)配置。</p><p>　　接受 IP 地址塊分配的系統(tǒng)管理員可以用輕松符合這些地址的方式管理未經(jīng)組織的網(wǎng)絡(luò)。例如，您有一個(gè)廣域網(wǎng)，在該廣域網(wǎng)中，TCP/IP 路由器連接的三個(gè)網(wǎng)絡(luò)（位于不同城市）中有 150 個(gè)主機(jī)。這三個(gè)網(wǎng)絡(luò)中的每個(gè)網(wǎng)絡(luò)都有 50

96、 個(gè)主機(jī)。向您分配了 C 類網(wǎng)絡(luò) 192.168.123.0。（此地址是為了便于說(shuō)明，實(shí)際上來(lái)自 Internet 中未分配的范圍。）這意味著可以將地址 192.168.123.1 至 192.168.123.254 用于您的 150 個(gè)主機(jī)。</p><p>　　在示例中不能使用的兩個(gè)地址為 192.168.123.0 和 192.168.123.255，因?yàn)橹鳈C(jī)部分全部為一和全部為零的二進(jìn)制地址無(wú)效。零地址無(wú)

97、效的原因是使用它在未指定主機(jī)的情況下指定了網(wǎng)絡(luò)。255 地址（在二進(jìn)制表示法中全部為一的主機(jī)地址）用來(lái)向網(wǎng)絡(luò)中的每個(gè)主機(jī)廣播消息。只需記住，任何網(wǎng)絡(luò)或子網(wǎng)中的第一個(gè)地址和最后一個(gè)地址不能分配給任何單獨(dú)主機(jī)。</p><p>　　現(xiàn)在，您應(yīng)該能夠?qū)?IP 地址給予 254 個(gè)主機(jī)。如果所有 150 臺(tái)計(jì)算機(jī)都在一個(gè)網(wǎng)絡(luò)中，則此操作會(huì)進(jìn)行得很順利。但是，您的 150 臺(tái)計(jì)算機(jī)位于三個(gè)單獨(dú)的物理網(wǎng)絡(luò)中。不用為每個(gè)網(wǎng)絡(luò)請(qǐng)

98、求更多地址塊，只需將網(wǎng)絡(luò)分成使您可以在多個(gè)物理網(wǎng)絡(luò)中使用一個(gè)地址塊的子網(wǎng)。</p><p>　　在此情況下，使用使網(wǎng)絡(luò)地址更大和可能的主機(jī)地址范圍更小的子網(wǎng)掩碼將網(wǎng)絡(luò)分成四個(gè)子網(wǎng)。也就是說(shuō)，您正在“借用”某些通常用于主機(jī)地址的位，并將它們用于地址的網(wǎng)絡(luò)部分。子網(wǎng)掩碼 255.255.255.192 給予您四個(gè)網(wǎng)絡(luò)，每個(gè)網(wǎng)絡(luò)有 62 個(gè)主機(jī)。由于用二進(jìn)制表示 255.255.255.192 與 1111111.11

99、111111.1111111.11000000 相同，所以此操作可以順利進(jìn)行。最后一個(gè)八位組的前兩位數(shù)字變?yōu)榫W(wǎng)絡(luò)地址，所以您獲得了附加網(wǎng)絡(luò) 00000000 (0)、01000000 (64)、10000000 (128) 和 11000000 (192)。（某些管理員只使用將 255.255.255.192 作為子網(wǎng)掩碼的其中兩個(gè)子網(wǎng)。有關(guān)此主題的更多信息，請(qǐng)參閱 RFC 1878。）在這四個(gè)網(wǎng)絡(luò)中，最后 6 個(gè)二進(jìn)制數(shù)字可用于主機(jī)地

100、址。</p><p>　　使用子網(wǎng)掩碼 255.255.255.192，則您的 192.168.123.0 網(wǎng)絡(luò)變?yōu)樗膫€(gè)網(wǎng)絡(luò) 192.168.123.0、192.168.123.64、192.168.123.128 和 192.168.123.192。這四個(gè)網(wǎng)絡(luò)將擁有以下有效主機(jī)地址：</p><p>　　192.168.123.1-62</p><p>　　192

101、.168.123.65-126</p><p>　　192.168.123.129-190</p><p>　　192.168.123.193-254</p><p>　　再次重審，全部為一或全部為零的二進(jìn)制主機(jī)地址無(wú)效，所以您不能使用最后一個(gè)八位組為 0、63、64、127、128、191、192 或 255 的地址。</p><p>　　

102、現(xiàn)在，看兩個(gè)主機(jī)地址 192.168.123.71 和 192.168.123.133，您就會(huì)了解其原理。如果使用默認(rèn) C 類子網(wǎng)掩碼 255.255.255.0，則這兩個(gè)地址都位于 192.168.123.0 網(wǎng)絡(luò)中。但是，如果使用子網(wǎng)掩碼 255.255.255.192，則它們位于不同網(wǎng)絡(luò)中；192.168.123.71 位于 192.168.123.64 網(wǎng)絡(luò)中，192.168.123.133 位于 192.168.123.128