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Bạn có thể tùy chỉnh hồ sơ cá nhân người dùng mặc định để kiểm soát các tùy chọn và cài đặt một người dùng mới sẽ nhận được. Sửa đổi hồ sơ cá nhân người dùng mặc định sẽ chỉ ảnh hưởng đến hồ sơ của người sử dụng mới; hồ sơ cá nhân hiện có sẽ không được | 13-4 Chapter 13 Supporting TCP IP Table 13-1 IP Address Classes Class Network ID Range of First Octet Number of Available Network Segments Number of Available Hosts Subnet Mask A w.0.0.0 1-126 126 16 777 214 255.0.0.0 B w.x.0.0 128-191 16 384 65 534 255.255.0.0 C w.x.y.0 192-223 2 097 152 254 255.255.255.0 D N A 224-239 N A N A N A E N A 240-255 N A N A N A Classes A B and C are available for registration by public organizations. Actually most of these addresses were snapped up long ago by major companies and Internet service providers ISPs so the actual assignment of an IP address to your organization will likely come from your chosen ISP. Classes D and E are reserved for special use. The address class determines the subnet mask used and therefore determines the division between the network ID and the host ID. For class A the network ID is the first octet in the IP address for example the 98 in the address 98.162.102.53 is the network ID . For class B it is the first two octets and for class C it is the first three octets. The remaining octets not used by the network ID identify the host ID. 1 Exam Tip Remember the IP address ranges that fall into each class and the default subnet mask for each class. This information not only helps to determine how a classful IP addressing scheme will apply to a situation but also how to customize a scheme using the classless method discussed next . Classless Interdomain Routing CIDR In the classful method of IP addressing the number of networks and hosts available for a specific address class is predetermined by the default subnet mask for the class. As a result an organization that is allocated a network ID has a single fixed network ID and a specific number of hosts. With the single network ID the organization can have only one network connecting its allocated number of hosts. If the number of hosts is large the network cannot perform efficiently. To solve this problem the concept of classless interdomain routing CIDR was .