Chúng ta hãy giả sử rằng địa chỉ IP của một hệ thống phải được thay đổi bởi vì hệ thống được chuyển đến một subnet khác nhau. Trong trường hợp này, thông tin và phát sóng mặt nạ mạng vẫn giữ nguyên. Việc chuyển sang mạng mới liên quan đến việc thay đổi địa chỉ IP của giao diện duy nhất. | 330 UNIX System Administration A Beginner s Guide Let s suppose that the IP address of a system must be changed because the system is moved to a different subnet. In this case the netmask and broadcast information remain the same. The move to the new network involves changing the IP address of the interface only. The old IP address is and the new IP address is . The following command would be used to change the network information on the fly ifconfig eth0 To make this change permanent we must modify the etc hosts file. This file contains the mapping between hostname and the associated IP address. On system startup the IP address is derived from the file and applied to the interface. The netmask and broadcast information are the same we can use the existing values. The hostname could also be specified on the command line instead of the IP address. Thus the command ifconfig eth0 fred accomplishes the same result assuming that fred has been assigned the IP address of either in the etc hosts file DNS or the NIS hosts database. As you can see changing the IP address for an interface is relatively straightforward. However changing other interface characteristics requires a bit more work. To extend the preceding example let us now assume that we must change the netmask and broadcast information. To change the interface the administrator could use ifconfig eth0 netmask broadcast In the example above the netmask and broadcast keywords must be used to identify the information that follows each keyword. The netmask contains 1s in the bit positions of the 32-bit address that are to be used for the network and subnet if appiicable parts and Os for the host o rtinn. The netmasldsubnet portion must occupy at least as many bits as is standard for the particular network class. If no subnete are defirted die stnndrrd hetmask is ssed. Wlra ssing subhets they require that more bits than what is
