TAILIEUCHUNG - CCIE Professional Development Large-Scale IP Network Solut phần 2

Trước khi hoàn toàn giải thích làm thế nào subnetting được thực hiện, nó là cần thiết để xác định mặt nạ mạng con. Nhớ lại rằng phần mạng của địa chỉ không thể thay đổi. Đối với một router để quyết định những gì một phần của địa chỉ mạng và phần nào là chủ nhà, | Before fully explaining how subnetting is performed it is necessary to define subnet masking. Recall that the network portion of the address cannot be changed. For a router to decide what part of the address is the network and what part is the host a 32-bit number is used to mask out each portion. This mask performs a logical AND operation with the IP address. Wherever the mask is binary one it is considered the network portion when the mask is zero it is considered the host. A zero in the network portion tells the router not to consider this part of the network during routing decisions see Figure 2-5 . Figure 2-5. Subnetting Using the Logical AND Operation Decimal Binary Address Mask 1111 1111 1111 1111-0000 0000-00000000 If you want to do subnetting you can mask the host portion for the router. If you want to use eight bits from the host portion of the address then this leg-bit masking at this point we will have 254 subnets for the class B network Decimal Binary Address 1000 0000 Mask 1111 0000 Subnetting is performed by borrowing bits from the host portion of the address and then using them to indicate different segments on the network. A network administrator can borrow any number of bits from the host portion as long as a few bits are left available to assign host addresses. NOTE Subnetting extends the network portion of the assigned address to accommodate all physical segments of a network. Subnetting Example In network suppose that you want to perform eight-bit masking of the class B network. Recall that the first two bytes cannot be altered in a class B network. Therefore to perform subnet masking you must borrow bits from the third byte and use it completely. As mentioned earlier subnet masking is performed by borrowing bits from the host portion of the assigned network address which assigns more .

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