What is the subnet mask for a Class A network?

The subnet mask of Class A network is "255.255.255.0". Subnet mask is a virtual IP technology produced in order to solve the problem of IP address allocation under the background of shortage of IPv4 address resources. The subnet mask divides the three types of addresses A, B, and C into several subnets, thus significantly improving the efficiency of IPv4 address allocation. IP address allocation efficiency; the default subnet mask of Class A is "255.0.0.0", the default subnet mask of Class B is "255.255.0.0", and the default subnet mask of Class C is "255.255.255.0" .

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

Subnet mask is also called network mask, address mask, and subnet mask. It is used to indicate which bits of an IP address identify the subnet where the host is located, and which bits The bit identifies the host's bit mask. The subnet mask cannot exist alone, it must be used in conjunction with the IP address.

The subnet mask is a 32-bit address that is used to mask a part of the IP address to distinguish the network identification and the host identification, and to indicate whether the IP address is on a LAN or a WAN.

Subnet mask is a virtual IP technology produced to solve the problem of IP address allocation under the background of shortage of IPv4 address resources. The three types of addresses A, B, and C are divided into several subnets through the subnet mask. , thus significantly improving the efficiency of IP address allocation and effectively solving the situation of tight IP address resources.

1. Class A default subnet mask: 255.0.0.0

2. Class B default subnet mask: 255.255.0.0

3, Category default subnet mask: 255.255.255.0

The subnet mask consists of consecutive 1 and 0. The consecutive 1 represents the network address and the consecutive 0 represents the host address. Through 0 The number of can be used to calculate the capacity of the subnet (the IP address range of the hosts in the subnet). First, let’s take a look at how the default subnet mask 255.255.255.0 divides subnets. The binary number of this subnet mask consists of 24 1s and 8 0s. The 8 0s represent the subnets divided by the subnet mask. The subnet capacity is 256 (2 to the 8th power), which means that 192.168.1.0-255 are all in the same subnet. There are only 254 available addresses among these 256 addresses because the first IP address of each subnet is specified. is the network segment address, and the last IP address is the broadcast address, both of which are unavailable. For example: For the network segment 192.168.1.0, if the subnet mask is set to 255.255.255.0, 192.168.1.1-192.168.1.254 is the available IP address. If the IP address is set in this range, the computers can connect to each other normally.

Subnet masks are divided into two categories:

1. Default (Automatically generated) Subnet mask: The default subnet mask is not divided into subnets, the corresponding network number bits are all set to 1, and the host number is set to 0.

(1), Class A network default subnet mask: 255.0.0.0

(2), Class B network default subnet mask: 255.255.0.0

(3), Class C network default subnet mask: 255.255.255.0

2. Custom subnet mask: It divides a network into several subnets, and each segment needs to use a different subnet mask. Network number or subnet number, in fact we can think of it as dividing the host number into two parts: subnet number and subnet host number. The format is as follows:

(1), IP address without subnetting: network number, host number

(2), IP address after subnetting: network number, subnet number Subnet host number

That is to say, after the IP address is divided into subnets, part of the previous host number position is given to the subnet number, and the rest is the subnet host number.

The subnet mask is a 32-bit binary number, and its subnet host identification part is all "0". You can use the subnet mask to determine whether two hosts are on the same subnet. If the IP addresses of two hosts are ANDed with their subnet masks and the result is the same, it means that the two hosts are in the same subnet.

Extended knowledge: The function of subnet mask

The subnet mask is a 32-bit address, which is related to the IP address A technique used in combination. It has two main functions. One is to shield part of the IP address to distinguish the network identification and the host identification, and to indicate whether the IP address is on the local area network or on the remote network. The second is used to divide a large IP network into several small sub-networks.

Using subnets is to reduce the waste of IP. Because with the development of the Internet, more and more networks are created, some with hundreds of networks, and some with only a few. This wastes a lot of IP addresses, so subnets must be divided. Using subnets can improve the efficiency of network applications.

The method to determine whether two computers belong to the same network segment through the computer's subnet mask is to convert the computer's decimal IP address and subnet mask into binary form, and then perform binary AND ) calculation (all 1s result in 1, incomplete 1s result in 0), if the results are the same, then the two computers belong to the same network segment.

1. Declare the network address and host address

The subnet mask must be used in conjunction with the IP address. Table 1 shows the binary and decimal correspondence between the default subnet masks of commonly used network Class A, B, and C IP addresses. The working process of the subnet mask is: perform a bitwise logical AND operation on the 32-bit subnet mask and the IP address in binary form to obtain the network address, and combine the binary non-result of the subnet mask with the IP address. The binary logical AND operation is performed, and the result obtained is the host address. For example: 192.168.10.11 AND 255.255.255.0, the result is 192.168.10.0, which means: the IP address belongs to the network 192.168.10.0, and its host number is 11, that is, the host number 11 in this network.

2. Divide subnets

The subnet mask mechanism provides a method of subnet division. Its function is to: reduce the traffic on the network; save IP addresses; facilitate management; and solve certain problems of the physical network itself. After dividing subnets using subnet masks, communication can be achieved within the subnet, but communication across subnets cannot be achieved. Routers should be used for inter-subnet communication, and static routing information should be configured correctly. To divide subnets, you should follow the rules of subnet division structure. It is to use continuous 1 to increase the number of digits in the IP address to represent the network address, and at the same time reduce the number of digits to represent the host address. For example, the IP address is 130.39.37.100, the network address is 130.39.37.0, the subnet address is 130.39.37.0, the subnet mask is 255.255.255.0, the network address part and the subnet identification part correspond to "1", and the host identification part corresponds to "0". Using CIDR, it is expressed as: 130.39.37.100/24, which is the IP address/mask length. The 8-bit binary value corresponding to 255 on the third byte is the digits of the host address lent to the network address part, which serves as the number of digits to divide the subnet.

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