计算机英语基础知识 C

计算机英语基础知识 C
Subnetting

　　A 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
192.168.123.193-254

　　Remember, 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.

　　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，所以您获得了附加网络 00000000 (0)、01000000 (64)、10000000 (128) 和 11000000 (192)。（某些管理员只使用将 255.255.255.192 作为子网掩码的其中两个子网。有关此主题的更多信息，请参阅 RFC 1878。）在这四个网络中，最后 6 个二进制数字可用于主机地址。

　　使用子网掩码 255.255.255.192，则您的 192.168.123.0 网络变为四个网络 192.168.123.0、　192.168.123.64、192.168.123.128 和 192.168.123.192。这四个网络将拥有以下有效主机地址：
192.168.123.1-62
192.168.123.65-126
192.168.123.129-190
192.168.123.193-254

　　再次重审，全部为一或全部为零的二进制主机地址无效，所以您不能使用最后一个八位组为 0、63、64、127、128、191、192 或 255 的地址。

　　现在，看两个主机地址 192.168.123.71 和 192.168.123.133，您就会了解其原理。如果使用默认 C 类子网掩码 255.255.255.0，则这两个地址都位于 192.168.123.0 网络中。但是，如果使用子网掩码 255.255.255.192，则它们位于不同网络中；192.168.123.71 位于 192.168.123.64 网络中，192.168.123.133 位于 192.168.123.128 网络中。