IP Calculation Method

Source: Internet
Author: User

IP address and subnet mask

We all know that IP addresses are composed of four digits. Here, let's take a look at three common types of IP addresses.

Class a ip segments to

Class B IP segments to

C-type IP segments to

The default subnet mask allocated by XP is only 255 or 0 for each segment.

Class A default subnet mask A subnet can accommodate up to 16.77 million computers

Default subnet mask of type B a subnet can accommodate a maximum of 60 thousand computers

Class C default subnet mask a subnet can accommodate a maximum of 254 computers

I used to think that if you want to put some computers in the same network segment, you only need to make the first three paragraphs of the IP address the same. Today, I know that I am wrong. If so, can a single subnet only accommodate 254 computers? It's a joke. Let's take a look at it in detail.

If you want to be in the same network segment, as long as the network ID is the same, what should you do with the network ID? The first thing to do is to convert the IP address of each segment to binary. (Some people say that I won't change it. It doesn't matter. We can use the built-in calculator in Windows. Open the calculator, click View> science type, enter a decimal number, and then click the binary "Single-Choice Point to switch to the binary .? Br>

After switching the subnet mask to binary, we will find that all the subnet masks are composed of a string of consecutive 1 and a string of consecutive 0 (a total of 4 segments, each segment of 8 bits, 32 digits in total ). 11111111.20..00000000.000000000000 11111111.111111.

201710000255.0 11111111.11111111.111111.00000000

This is the binary form of the default subnet mask of Class A/B/C. In fact, there are many subnet masks, as long as it is a string of continuous 1 and a string of continuous 0 (each segment is 8 bits ). For example, 11111111.111111.11111000.00000000, which is also a valid subnet mask. The subnet mask determines the number of computers in a subnet. The computer formula is 2 to the power of m. Here, we can see m as the number of digits following 0. For example, when is converted to a binary value, it is 11111111.1111.1111.00000000. There are 8 digits behind it, and m is 8. The subnet mask can accommodate 2 Power 8 computers, that is, 256 machines, but two IP addresses are not available, that is, the last segment cannot be 0 or 255. If the two IP addresses are subtracted, the number is 254. Let's try again.

201710000248.0 how many computers can this subnet mask accommodate?

Calculation method:

Convert it to a four-digit binary number (each segment is 8 bits. If it is 0, it can be written as 8 zeros, that is, 00000000)


Then, there are several zeros behind the number, and there are 11 in total, that is, the power of 2's 11, equal to 2048. This subnet mask can accommodate a maximum of 2048 computers.

The maximum number of computers that a subnet can accommodate. Let's look at the question of reverse algorithms.

A company has 530 computers and forms a peer-to-peer lan. What is the most suitable subnet mask?

First of all, it is undoubtedly the most suitable class B ip address for 530 computers (class A, too many, and class C is not enough, it must be Class B ), however, the default subnet mask of Class B is 255.0.0, which can accommodate 60 thousand computers. Obviously, this is not suitable. How many subnet masks are suitable? Let's first list a formula.

2 to the power of m = 560

First, we confirm that 2 must be greater than the power of 8, because we know that the power of 2 is 256, that is, the maximum number of computers allowed by Class c ip addresses, we try to use the power of 9 to the power of 2 to the power of 512, less than 1024, and the power of 10 to 2 is. It seems that the power of 2 is the most suitable. The subnet mask consists of 32 bits. It has been determined that the next 10 bits are 0, and the first 22 bits are 1. The most suitable subnet mask is 11111111.111111.11111100.00000000, which is converted to 10 hexadecimal format, that is

Assign and calculate the subnet mask. Next, let's look at the IP address CIDR block.

I believe many people share the same relationship with me. I think that the IP address is in the same network segment as long as the first three paragraphs are the same. In fact, this is not the case. Similarly, I convert each segment of the IP address into a binary number, take IP Address:, subnet mask: as an example.


(Here, we will describe that, like the subnet mask, each segment has eight bits. If there are less than eight bits, add 0 to the front .)

IP 112.1600.10101000.20.0000.00000001

Subnet Mask 11111111.111111.1111111111.00000000

Here, I would like to explain how the same network segment is considered.

In order to be in the same network segment, the network ID must be the same. How can this problem be solved? The network identification algorithms of various IP addresses are different. Class A is only the first segment. Class B, only the first and second sections. Class C: calculates the first, second, and third sections.

The algorithm only needs to set the IP address AND subnet mask each digit AND.

AND method: 0 AND 1 = 0 0 AND 0 = 0 1 AND 1 = 1

For example: And,, first converted to binary, AND each And

IP 112.1600.10101000.20.0000.00000001

Subnet Mask 11111111.111111.1111111111.00000000

Obtain the AND result 112.1600.10101000.00000000.00000000.

Convert to in decimal format. This is the network ID,

Then, the subnet mask is reversed, that is, the IP address AND

The result is calculated as follows: 2017.hangzhou.hangzhou.00000001, which is converted to a 10-digit system, that is,, indicates the host ID. To be in the same network segment, you must have the same network ID.

Let's take a look at the Class B IP address changed to the default subnet mask.

For example, IP Address:,, all subnet masks are set to Are they in the same network segment?

First convert these to binary 101100.10111100. 10111100.10111100.00000101.11011010

201710000254.0 11111111.111111.11111110.00000000

Respectively AND, get



The network IDs are different, that is, they are not in the same CIDR block.

Check whether the IP address is in the same CIDR block. Now, let's look at it.

A company has 530 computers to form a peer-to-peer lan. What is the most suitable subnet mask and IP address?

The subnet mask is not mentioned. The previous calculation result is 11111111.1111111100.00000000, that is,

Now we need to determine how to allocate IP addresses. First, select a Class B IP segment. Here we will select 188.188.x.x.

In this way, the key to determining the first two IP segments is to determine the third segment, as long as the network ID is the same. Let's first determine the network number. (We set? Right up, 0 corresponds to *, as shown below :)

201710000252.0 11111111.111111.11111100.00000000

188.188.x.x 10111100.10111100 .?????? **.********

Network ID 10111100.10111100 .?????? 00.00000000

From this we can see ,? (Only 0 and 1 can be entered. Not all are 0 and 1.) Let's fill in 0, *. In this case, our IP address is

10111100.101100.000000 **. *********, a total of 530 computers, the last segment of the IP address 1 ~ 254 can be allocated to 254 computers, 530/254 = 2.086, using the method of entering 1, get an integer of 3, so we have determined that the third segment of the IP address should be divided into three different numbers, that is, set ** in 000000 ** to three numbers. You can only enter 1 and 0, and the numbers are different each time. As for what to fill in, it will be convenient, such as 00000011, convert it to binary, which is 1, 2, 3, so that the third segment is also determined. In this way, you can divide the IP address into 188.188.1.y, 188.188.2.y, 188.188.3.y, and y ~ Within the range of 254, And the 530 computers are different from each other's IP addresses.

Here, we will list the conversion tables of the number of network subnets and subnet masks in class A, B, and C for your reference.


Number of subnets occupied by the number of digits subnet mask number of hosts in the subnet

2 1 Listen 128.0.0 8,388,606

4 2 listen 192.0.0 4,194,302

8 3 2,097,150

16 4 1,048,574

32 5 524,286

64 6 262,142

128 7 131,070

128 8 65,534

Class B:

Number of subnets occupied by the number of digits subnet mask number of hosts in the subnet

2 1 100000000128.0 32,766

4 2 faster than 192.0 16,382

8 3 running 255.224.0 8,190

16 4 4,094

32 5 running 255.248.0 2,046

64 6 1,022

128 7 510

256 8 254

Class C:

Number of subnets occupied by the number of digits subnet mask number of hosts in the subnet

2 1 Gbit/s 2017100000000128 126

4 2 rows without errors 192 62

8 3 running 30

16 4 Tib 201710000240 14

32 5 bytes 255.255.248 6

64 6 bytes 255.255.252 2

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