Among the Cisco Network Technology College CCNA Teaching and Exam, many students are always the calculation of the small pain and mask when conducting IP address planning. Now give everyone a tip, you can successfully solve this problem.
First, let's see a common type in a CCNA test: a host's IP address is 202.112.14.137, the mask is 255.255.255.224, requiring the network address and broadcast address of the network where this host is located.
A conventional approach is to convert this host address and subnet mask into binary numbers, and the network can get the network address after logic and operation. In fact, everyone can get another way to get another method: 255.255.255.224 The IP address accommodated by 256-224 = 32 (including network addresses and broadcast addresses), then the network address with this mask must be It is a multiple of 32. The network address is the beginning of the subnet IP address, the broadcast address is end, and the host address that can be used is within this range, so only 128 of the multiple of 32 and 32, so the network address is 202.12.14.128. The broadcast address is a network address of the next network. The next 32 multiples are 160, so the broadcast address can be obtained as 202.112.14.159. In the CCNA test, there is also a type, you want you to make subnet addresses and calculate subnet masks based on the number of hosts of each network. This can also be calculated as described above. For example, a subnet has 10 hosts, then 10 1 1 1 = 13 IP addresses are required for this subnet. (Note that the first 1 refers to the gateway address required for this network connection, and the two 1 of the two 1 refer to the network address and broadcast address.) 13 less than 16 (16 equal to 2), the host The bit is 4 digits. 256-16 = 240, so the subnet mask is 255.255.255.240.
If a subnet has 14 hosts, many of the frequent mistakes of many students are: still allocating subnets with 16 address space, and forgetting to assign addresses to the gateway. This is wrong, because 14 1 1 1 = 17, greater than 16, so we can only assign subnets with 32 addresses (32 equal to 2) space. At this time, the subnet mask is: 255.255.255.224.
