tailieunhanh - sams teach yourself tcp ip in 24 hours phần 3

Mặt nạ mạng con phải được tính toán cẩn thận và phải phản ánh việc tổ chức nội bộ của mạng. Tất cả các host trong một subnet có cùng một subnet ID và subnet mask. Vì lợi ích của người dân, mặt nạ mạng con thường được biểu diễn trong ký hiệu chấm thập phân tương tự như các ký hiệu được sử dụng cho một địa chỉ IP. | 74 HOUR 5 Subnetting and CIDR Subnet masks must be carefully calculated and must reflect the internal organization of the network. All the hosts within a subnet should have the same subnet ID and subnet mask. For the benefit of people the subnet mask is usually expressed in dotted decimal notation similar to the notation used for an IP address. As you ll recall from the preceding section the subnet mask is a 32-bit binary number. You can convert the binary subnet mask to a dotted decimal address using the address conversion techniques described in Hour 4. A subnet mask is usually much easier to convert to dotted decimal format than an IP address. The subnet mask bits representing the IP address s network ID and the subnet ID are 1 bits. The bits representing the IP address s host ID are 0 bits. This means that with a few rare and bewildering exceptions the 1 bits are all on the left and the 0 bits are all on the right. Any full octet of 1s in the subnet mask will appear as 255 binary 11111111 in the dotted decimal subnet mask. Any full octet of 0s will appear as 0 binary 00000000 in the subnet mask. Hence the common subnet mask 11111111111111111111111100000000 is expressed in dotted decimal notation as . Likewise the subnet mask 11111111111111110000000000000000 is expressed in dotted decimal notation as . As you can see it is easy to determine the dotted decimal equivalent of a subnet mask that divides the address at an octet boundary. However some subnet masks do not divide the address at an octet boundary. In that case you must simply determine the decimal equivalent of the mixed octet the octet containing both 1s and 0s . To convert a binary subnet mask to dotted decimal notation follow these steps 1. Divide the subnet mask into octets by writing the 32-bit binary subnet mask with periods inserted at the octet boundaries 2. For every all-ones octet write down 255. For every all-zeros octet write down 0.