The success of TCP(论文前言翻译,实用).doc

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. 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 to divide an IP address into two parts. One part identifies the host (computer), the other part identifies the network to which it belongs. To better understand how IP addresses and subnet masks work, look at an IP (Internet Protocol) address and see how it is organized. IP addresses: Networks and hostsAn 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.IP addresses are normally expressed in dotted-decimal format, 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.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 eight binary digits.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 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.For a TCP/IP wide area 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 only 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 hosts network. After the packet is delivered to the destinations network, the packet is delivered to the appropriate host.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. If you take the example 192.168.123.132 and divide it into these two parts you get the following: 192.168.123. Network .132 Host-or- 192.168.123.0 - network address. 0.0.0.132 - host address.Subnet maskThe 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.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 supplied 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 subnet mask is: 11111111.11111111.11111111.0000000Lining up the IP address and the subnet mask together, the network and host portions of the address can be separated: 11000000.10101000.01111011.10000100 - IP address (192.168.123.132) 11111111.11111111.11111111.00000000 - Subnet mask (255.255.255.0)The first 24 bits (the number 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: 11000000.10101000.01111011.00000000 - Network address (192.168.123.0) 00000000.00000000.00000000.10000100 - Host address (000.000.000.132)So now you know, for this example 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 destination address of 192.168.123.132, your computer will receive it from the network and process it.Almost all decimal subnet masks convert to binary numbers that are all ones on the left and all zeros on the right. Some other common subnet masks are: Decimal Binary 255.255.255.192 1111111.11111111.1111111.11000000 255.255.255.224 1111111.11111111.1111111.11100000Internet RFC 1878 describes the valid subnets and subnet masks that can be used on TCP/IP networks. Network classesInternet addresses are allocated by the InterNIC, the organization that administers the Internet. These 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. You 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 an example address: ?Class A networks use a default subnet mask of 255.0.0.0 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.?Class B networks use a default subnet mask of 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.?Class C networks use a default subnet 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.In some scenarios, 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 restrictions. The next section explains how networks can be divided using subnet masks. SubnettingA Class A, B, or C TCP/IP network can be further 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 the real world.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 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 illustration, this address is actually from a range that is not allocated on the Internet.) This means that you can use the addresses 192.168.123.1 to 192.168.123.254 for your 150 hosts.Two addresses that cannot be used in your example are 192.168.123.0 and 192.168.123.255 because binary 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 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 host.You should now 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 each network, you divide your network into subnets that enable you to use one block of addresses on multiple physical networks.In this case, you divide your network into four subnets by using a subnet 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 network 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 two digits of the last octet become network addresses, so you get the additional networks 00000000 (0), 01000000 (64), 10000000 (128) and 11000000 (192). (Some administrators will only use two of the subnetworks using 255.255.255.192 as a subnet mask. For more information on this topic, see RFC 1878.) In these four networks, the last 6 binary digits can be used for host addresses.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 valid host addresses: 192.168.123.1-62 192.168.123.65-126 192.168.123.129-190 192.168.123.193-254Remember, again, 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.You can see how this works by looking 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 255.255.255.192, 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. Default gatewaysIf 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 links the hosts 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 network.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 own IP address. The result of this comparison tells the computer whether the destination is a local host or a remote host.If the result of this process determines the destination to be a local host, 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 defined in its TCP/IP properties. It is then the responsibility of the router to forward the packet to the correct subnet. TroubleshootingTCP/IP network problems are often caused by incorrect configuration of the three main entries in a computers TCP/IP properties. By understanding how errors in TCP/IP configuration affect network operations, you can solve many common TCP/IP problems.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 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 255.255.255.0 in your TCP/IP configuration, hosts will not be able to determine that some computers are on different subnets than their own. When this happens, packets destined for hosts on different physical networks that are part of the same Class C address will not be sent to a default gateway for delivery. A common symptom of this is when a computer can communicate with hosts that are on its local network and can talk to all remote networks except those that are nearby and have the same class A, B, or C address. To fix this problem, just enter the correct subnet mask in the TCP/IP configuration for that host.Incorrect IP Address: If you put computers with IP addresses that should be on separate 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 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 physical network have IP addresses on the same IP subnet. If you run out of IP addresses on a single network segment, there are solutions that go beyond the scope of this article.Incorrect 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. 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 organization has a router to an internal TCP/IP network and another router connected to the Internet. 作为 Internet 的网络协议，TCP/IP 的成功在很大程度上归功于它将不同大小的网络和不同类型的系统连接在一起的能力。这些网络被强制定义为具有预定义大小的三个主要类（还有其他一些类别），每一类都可以由系统管理员分成更小的子网。子网掩码用于将 IP 地址分成两个部分。一部分标识主机（计算机），另一部分标识它所属的网络。查看 IP（Internet 协议）地址并研究它的组织方式可以帮助您更好地理解 IP 地址和子网掩码的工作方式。 IP 地址：网络和主机IP 地址是一个 32 位数字，它唯一地标识 TCP/IP 网络上的主机（计算机或其他设备，如打印机或路由器）。IP 地址通常以点分十进制格式表示，四个数字由句点分隔，例如 192.168.123.132。要了解子网掩码如何用于区分不同的主机、网络和子网，请查看以二进制表示的 IP 地址。例如，点分十进制 IP 地址 192.168.123.132 在二进制表示法中为 32 位数字 110000000101000111101110000100。此数字的含义可能很难理解，因此它被分成四个部分，每个部分有八个二进制数字。这些八位部分称为八位组。示例 IP 地址于是变成 11000000.10101000.01111011.10000100。此数字表示的含义只是稍微明白了一点，因此在大多数应用中，会将二进制地址转换为点分十进制格式 (192.168.123.132)。点分十进制数字是从二进制转换为十进制表示法的八位组。为了让 TCP/IP 广域网 (WAN) 以一个网络集的形式高效工作，在网络间传递数据包的路由器并不知道信息包的目标主机的确切位置。路由器只知道主机是哪一个网络的成员，并使用存储在路由表中的信息来确定如何将数据包送达目标主机的网络。当数据包被传送到目标网络后，该数据包就会被传送到相应的主机。为了让此过程顺利进行，IP 地址分为两个部分。IP 地址的前一部分作为网络地址，后一部分作为主机地址。以 192.168.123.132 为例，将它分为这两个部分之后，会得到： 192.168.123. 网络 .132 主机- 或 - 192.168.123.0 网络地址。0.0.0.132 主机地址。子网掩码第二项是子网掩码，它是 TCP/IP 正常工作所必需的。TCP/IP 协议使用子网掩码确定主机是在本地子网中还是在远程网络中。在 TCP/IP 中，将哪部分 IP 地址用作网络地址和主机地址并不固定，所以除非您掌握详细的信息，否则无法确定上述网络地址和主机地址。此信息在另一个 32 位数字中提供，称为子网掩码。在本例中，子网掩码为 255.255.255.0。如果您不知道二进制表示法中的 255 等于 11111111，可能并不清楚该数字表示的含义；照此分析，子网掩码为： 11111111.11111111.11111111.0000000将 IP 地址和子网掩码排列在一起比较，就可以分清该地址的网络部分和主机部分： 11000000.10101000.01111011.10000100 - IP 地址 (192.168.123.132)11111111.11111111.11111111.00000000 - 子网掩码 (255.255.255.0)前 24 位（子网掩码中的数字 1）被标识为网络地址，后 8 位（子网掩码中剩余的数字 0）被标识为主机地址。据此可以得到： 11000000.10101000.01111011.00000000 - 网络地址 (192.168.123.0)00000000.00000000.00000000.10000100 - 主机地址 (000.000.000.132)这样，我们就可以知道，在这个使用 255.255.255.0 子网掩码的示例中，网络 ID 为 192.168.123.0，主机地址为 0.0.0.132。当数据包到达 192.168.123.0 子网（从本地子网或远程网络），而且它的目标地址为 192.168.123.132 时，您的计算机将从网络接收它并对它进行处理。几乎所有十进制子网掩码都转换为左侧全部是一、右侧全部是零的二进制数字。其他一些常见的子网掩码有： 十进制 二进制255.255.255.192 1111111.11111111.1111111.11000000255.255.255.224 1111111.11111111.1111111.11100000Internet RFC 1878描述了可在 TCP/IP 网络中使用的有效子网和子网掩码。 网络类Internet 地址由管理 Internet 的机构 InterNIC来分配。这些 IP 地址分成若干类。其中最常见的是 A、B 和 C 类。也有 D 和 E 类，但是最终用户通常不会使用。每个地址类都有不同的默认子网掩码。可以通过查看 IP 地址的第一个八位组来识别该 IP 地址的类别。下面是 A、B 和 C 类 Internet 地址的范围，每一类地址都有一个示例： ?A 类网络使用的默认子网掩码为 255.0.0.0，第一个八位组为 0-127。地址 10.52.36.11 就是一个 A 类地址。它的第一个八位组为 10，介于 1 至 126 之间（包括 1 和 126）。?B 类网络使用的默认子网掩码为 255.255.0.0，第一个八位组为 128-191。地址 172.16.52.63 就是一个 B 类地址。它的第一个八位组为 172，介于 128 至 191 之间（包括 128 和 191）。?C 类网络使用的默认子网掩码为 255.255.255.0，第一个八位组为 192-223。地址 192.168.123.132 就是一个 C 类地址。它的第一个八位组为 192，介于 192 至 223 之间（包括 192 和 223）。在某些情况下，由于网络的物理拓扑或因为网络（或主机）的数目在默认的子网掩码限制之下并不适用，所以默认子网掩码值可能不适合机构的需要。下一部分将解释如何使用子网掩码划分网络。 子网配置系统管理员可以进一步划分 A、B 或 C 类 TCP/IP 网络或对这些网络进行子网配置。当您将 Internet 的逻辑地址结构（IP 地址和子网的抽象世界）与真实世界中使用的物理网络进行协调时，就有必要进行子网配置。接受 IP 地址块分配的系统管理员可以用轻松符合这些地址的方式管理未经组织的网络。例如，您有一个广域网，在该广域网中，TCP/IP 路由器连接的三个网络（位于不同城市）中有 150 个主机。这三个网络中的每个网络都有 50 个主机。向您分配了 C 类网络 192.168.123.0。（此地址是为了便于说明，实际上来自 Internet 中未分配的范围。）这意味着可以将地址 192.168.123.1 至 192.168.123.254 用于您的 150 个主机。在示例中不能使用的两个地址为 192.168.123.0 和 192.168.123.255，因为主机部分全部为一和全部为零的二进制地址无效。零地址无效的原因是使用它在未指定主机的情况下指定了网络。255 地址（在二进制表示法中全部为一的主机地址）用来向网络中的每个主机广播消息。只需记住，任何网络或子网中的第一个地址和最后一个地址不能分配给任何单独主机。现在，您应该能够将 IP 地址给予 254 个主机。如果所有 150 台计算机都在一个网络中，则此操作会进行得很顺利。但是，您的 150 台计算机位于三个单独的物理网络中。不用为每个网络请求更多地址块，只需将网络分成使您可以在多个物理网络中使用一个地址块的子网。在此情况下，使用使网络地址更大和可能的主机地址范围更小的子网掩码将网络分成四个子网。也就是说，您正在“借用”某些通常用于主机地址的位，并将它们用于地址的网络部分。子网掩码 255.255.255.192 给予您四个网络，每个网络有 62 个主机。由于用二进制表示 255.255.255.192 与 1111111.11111111.1111111.11000000 相同，所以此操作可以顺利进行。最后一个八位组的前两位数字变为网络地址，所以您