tailieunhanh - windows server 2008 tcp ip protocols and services microsoft 2008 phần 10

Subnetting được thiết kế để sử dụng hiệu quả hơn của một không gian địa chỉ cố định, cụ thể là một tiền tố địa chỉ IP. Các bit mạng cố định và các bit host biến. Ban đầu, các bit host được thiết kế để chỉ ID các máy trong một tiền tố địa chỉ IP. | Appendix A Internet Protocol IP Addressing 427 Table A-2 lists the ranges of host IDs based on the IP address classes. Table A-2 Address Class Ranges of Host IDs Address Class First Host ID Last Host ID Number of Hosts Class A 16 777 214 Class B 65 534 Class C 254 Subnets and the Subnet Mask Subnetting is designed to make more efficient use of a fixed address space namely an IP address prefix. The network bits are fixed and the host bits are variable. Originally the host bits were designed to indicate host IDs within an IP address prefix. With subnetting host ID bits can be used to express a combination of a subnetted address prefix and a new host ID thereby better utilizing the host bits. Consider a class B network that has 65 534 possible hosts. A network segment of 65 534 hosts is technically possible but impractical because of the accumulation of broadcast traffic. All nodes on the same physical network segment belong to the same broadcast domain and share the same broadcast traffic. Because making all 65 534 hosts share the same broadcast traffic is not a practical configuration most of the host IDs are not usable. To create smaller broadcast domains and make better use of the host bits RFC 950 defines a method of subdividing an address prefix into subnetworks subsets of the original classbased network by using bits in the host ID portion of the original IP address prefix. Each subnetwork or subnet is assigned a new subnetted address prefix. Hosts on subnets are assigned host IDs from the remaining host bits in the subnetted address prefix. Although RFC 950 discusses subnetting in terms of class-based address prefixes subnetting is a general technique that can be used on classless address prefixes or used recursively on subnetted address prefixes. This is described in the section Variable-Length Subnetting later in this appendix. The proper subnetting of an address prefix is transparent to the rest of