Một hồ sơ danh mục CIP cho cuốn sách này là có sẵn từ Thư viện Anh. Microsoft Press sách có sẵn thông qua các nhà sách và nhà phân phối trên toàn thế giới. Để biết thêm thông tin về phiên bản quốc tế, liên lạc với văn phòng Microsoft Tổng công ty địa phương của bạn hoặc liên hệ với Microsoft Press quốc tế trực tiếp | 40 Chapter 1 Understanding and Configuring IP not a unique address. The second part represents a specific 4-digit mailbox within the 5-digit ZIP code area a mailbox to which the post office represented by the ZIP code has the responsibility to deliver mail. However ZIP 4 addresses are much simpler than IPv4 addresses in one respect. When you look at a ZIP 4 address you know for certain which part of the address represents the post office the ZIP code and which part represents the individual mailbox the 4 . The dividing line between them never changes. The first five digits and the last four digits always have the same function. The tricky thing about IPv4 addresses is that the size of the network ID and the size of the host ID vary. Just by looking at an IPv4 address such as you cannot determine which of the 32 bits are used for the network ID and which are used for the host ID. To do this you need an additional piece of information. That piece of information is the subnet mask. Subnet Masks The subnet mask is used to determine which part of a 32-bit IPv4 address should be considered its network ID. For example when we write 24 the 24 represents the subnet mask and indicates that the first 24 of the 32 bits in that IPv4 address should be considered its network ID. For the IPv4 address shown in Figure 1-29 above the first 16 bits according to the picture are used for the network ID. Therefore the appropriate subnet mask to be used by a host assigned that address is 16. The two subnet masks we have just mentioned 16 and 24 are relatively easy to interpret. Because their values are divisible by 8 these subnet masks indicate that the network ID is composed of respectively the the first two complete octets and the first three complete octets of an IPv4 address. In other words the network ID of a host assigned the address 16 is and the host s network address is therefore . The network ID of a
