# What is a subnet mask

Source: Internet
Author: User
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How do I divide subnets? The first thing to memorize is 2 power: 2 of the 0 to 9 of the value of the square is: 1,2,4,8,16,32,64,128,256 and 512 respectively. Also have to understand is: The subnet division is to take away the host bit, this take away part as a subnet bit. So this means more subnets, fewer hosts.

The subnet mask is used to identify which part of the IP address is a network address. Which part is the host address, there are 1 and 0 components, long 32 bits, all 1 of bits represent the network number. Not all networks require subnets, so there are 1 concepts: The default subnet mask (defaulted subnet mask). The default subnet mask for class A IP addresses is 255.0.0.0; Class B is 255.255.0.0; The class C is 255.255.255.0.

CIDR is called inter-domain routing without class, ISPs often use this method to assign addresses to customers, and ISPs provide 1 blocks (block size) to customers, similar to this: 192.168.10.32/28, this row of numbers tells you how much your subnet mask is,/28 represents how many digits are 1, max/32. But what you have to know is that, whether it's a class A or B or other class address, the maximum available is only/30, that is, keep 2 digits to the main seat.

Several shortcuts for dividing subnets:

1. How many subnets will be generated by the subnet mask you choose?: 2 of x -2 (x for subnet bits, 2 for 1) PS: Here x is the subnet bit after the default mask is removed, For example, the network address 192.168.1.1, Mask 255.255.255.192, because it is a class C address, the mask is 255.255.255.0. Then 255.255.255.192 (x.x.x.11000000) uses two one as the subnet bit.

2. How many hosts can each subnet have?: 2 y-square -2 (Y represents the host bit, that is, the 2 to 0 portion)

3. Effective subnet is: Valid subnet number =256-10 subnet mask (result called block size or base numbers)

5. The effective host for each subnet is?: Ignoring the addresses of all 0 and all 1 in the subnet is the valid host address.

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