By default, a class B addressable network will allow up to 65,000 device addresses (host addresses). However, in reality, due to technological limitations, no single network can support such machines. Therefore, it is necessary to divide a single network into several subnets and this process is called subneting. In general, a subnet is a group of devices on the same network segment and shares the same subnet.
The need to divide into subnets In this example, a company is assigned a class B address, which can have up to 65,000 devices. However, existing network architectures have physical limitations on the number of hosts that can be connected, often smaller than the number of addresses that may be available in a Class B network. Moreover, the management on a network too many devices is also a great difficulty..jpg)
The benefits of subdividing into subnets
In addition to adding network addresses, dividing into subnets has the following benefits:
- Reduce network congestion by redirecting traffic and limiting the scope of broadcast messages.
- Limits within each subnet can occur (not affecting the entire LAN).
- Reduces CPU usage time by decreasing the traffic of your traffic
- Enhanced security (privacy policies may apply to individual subnets)
- Allows to apply different configurations on each subnet
Subnet mask
A subnet mask is a 32-bit number that identifies the network address portion of an IP address. There are two types of subnet masks: default subnet mask and custom subnet mask
Default Subnet Mask (Default Subnet Mask)
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To understand this concept, imagine a subnet mask as a net that covers the network address in an IP address. Each computer or router will use the subnet mask to determine the network address of the IP address it will send the message to. The bits that are not covered by the subnet mask are the bits that determine the device address in an IP address.
The bits in the subnet mask correspond to the network identification bits of the IP address equal to 1, the bits corresponding to the bits defining the device with a value of 0. In decimal form, if The network identifier of an IP address occupying a whole octet, the corresponding octet in the subnet mask will be 255.
If there is no subnet mask customization, the default subnet mask will be used to distinguish the network identification and device identification in an IP address.
Custom subnet mask
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The subnet address is the network address for a subnet. The subnet mask customization allows us to identify these subnet addresses in an IP address. When creating a subnet mask for a subnet, you also determine the maximum number of devices that can be connected in that subnet.
For example, imagine your network is assigned a class C address, but you need to divide it into subnets to improve the performance of the entire network. If you place a subnet mask as in the Layer C example shown above, your network can have up to 14 subnets (24-2) and each subnet can have up to 14 devices.
Most custom subnet masks cover the bits that are covered by the default subnet mask, but in addition to these bits, it extends a few more bits out of the next octets.
Like the default subnet mask, the subnet mask customization also includes bits 1, which correspond to the bits in the IP address covered by the subnet mask. In decimal form, each octet in the subnet mask that covers a whole octet in the IP address is also 255. The decimal value of the remaining octets in the subnet mask depends on the number of bits. is used to determine the subnet address.
Without a subnet mask customization, all the computers in your network must belong to the same physical network segment. With subnet masking, you can create more subnets. When you add a bit to the default subnet mask, you make that bit a bit part of the subnet mask, but that also reduces the remaining bits for the device address.
Admin IP address
Administering IP addresses in a TCP / IP network usually starts with obtaining a network address from an Internet Service Provider (ISP) or the organization responsible for allocating an Internet address. After having the network address, the following three important tasks must be completed to type the IP addresses for the devices on the network.
- Select the subnet mask
- Assign addresses to subnets
- Assign addresses to devices on the subnet
Select subnet mask
To identify the subnet mask, you must first determine the required subnet. This should be calculated based on the current status and expected growth of the company's network. Here are two ways you can use this to determine the subnet mask.
Option 1: Calculate subnet mask
Problem: Class C network address assignment 162.199.0.0 should be divided into 10 subnets. Value of the face how much subnet mask
In this example, we have a class B address that needs to be split into 10 subnets. To determine the subnet mask customization, follow these steps:
First, take the necessary number of subnets and convert that number into binary form. In this case, if you need 10 subnets, convert 10 to binary and 1010
Step 2, convert all bits in that binary value to 1. We will convert all bits of 1010 to 1 and add the zeroes to the result to get a full octet. The result is 11110000. Convert this binary value to decimal, which is 240. This is the extension (in addition to the default subnet mask) of the custom subnet mask. To get the custom subnet mask, just add this value after the default subnet mask 255255.0.0 and get 255.255.255.240.
We have a diagram and summary of steps as follows: 
Move this number to binary form
- Convert all bits to 1. Add the 0 bits to the following To be a full octet
Add the custom mask to the default subnet mask
Different custom customizations for each octet. Therefore, it is possible to set up a table to help us quickly determine the appropriate mask value. .jpg)
Continue counting until it reaches the rightmost column, at all bits c The octets are used in the subnet mask. It will be worth it is 255.
Next, determine the number of subnets corresponding to each subnet mask value. The number of subnets can be determined by the formula 2 ^ m-2, where m is the number of bits added to the subnet mask (in addition to the bits of the default mask). You need to subtract 2 because there are two addresses reserved per network. For example, if you use only 1 bit for the subnet mask (when the subnet mask value is 128), there will be 2 ^ 1-2 = 0 valid addresses for this subnet. If 2 bits are used for the subnet mask (subnet mask value is 192), there will be 2 ^ 2-2 valid values for the subnet mask. Let's continue with the next column.
The last step is to define the columns in the table that allow you to divide the network into the desired number of subnets. For example, if you need 8 subnets, select the column that allows up to 14 subnets, corresponding to a value of 240 in the subnet mask.
Note: In some cases, we must calculate the number of possible subnets with a network address and Subnet Mask Granted. This is the opposite of the problem. The steps are as follows:
- Transfer subnet mask to binary form
- Counts the number of bits that are fed into the custom subnet mask in addition to the bits of the default subnet mask, calling that number m
- Use the 2 ^ m-2 formula to calculate the number of subnets
Calculate the number of devices per subnet
After determining the value of the subnet mask, it is necessary to determine the number of devices that can be connected to each subnet.
Problem: How many devices can connect to the subnets in the following networks
- Class B network with 14 subnets and subnet mask 255.255.240.0
- To calculate the number of supported devices per subnet, move the subnet mask to binary and count the bits that have not been masked (that is, bits 0). Then use the following formula to calculate the maximum number of devices supported: 2 ^ u-2, where u is the number of 0s counted above.
For example, the 255.255.240.0 mask for Class B networks divides the network into 14 subnets. There will be 12 bits left unchecked. Applying the above formula we will calculate the maximum number of devices per subnetwork is 2 ^ 12-2 = 4094
We have a diagram and summary of steps as follows:
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- Transfer subnet mask to binary form
- Counts the bits that do not belong to the subnet mask
- Use the 2 ^ u-2 formula to calculate the number of devices per subnet
Assign a subnet address
Once you have identified subnet masks that match the number of subnetworks that need to be set up, you need to identify the addresses that will be assigned to each subnet.
Problem: List all items The valid subnet address for a Class B network is 131.56.0.0 with a subnet mask of 255.255.240.0.
In this example, you were given a class B address of 131.56.0.0, and you chose 255.255.240.0 as the subnet mask. To calculate the valid subnet addresses, first pass the mask value to the binary form. Find the rightmost bit 1 and convert that bit into a decimal form. In this example, the rightmost bit 1 has a decimal value of 16. This is called a progressive value
Next, create a list of subnet addresses by adding progressively to the provided network address. Note that the list will stop at a number equal to the value of the subnet mask.
| 131.56.0.0 |
To check the list, count the number of bits taken and o mask in the subnet mask and calculate the subnet mask according to company 2 ^ m-2. In this case, there are 4 b it is taken and o mask number of subnets is 2 ^ 4-2 = 14, equal to the number of subnets in the list above.
Assign a device address
The last step in the IP address management task is verify Assigns IP addresses that can be assigned to devices in each subnet. You need to know the subnet mask, subnet address, progressive value to calculate the initial address n and the last address on each subnet. Range e Valid IP addresses in each subnet are defined as follows:
Start address: By subnet address plus 1
- Address ending. By subnet address minus 2 (by current subnet address + progression number -2).
Problem: Identification Set the address range for the workstation n network subnet 131.56.32.0 Class B addresses 131.56.0.0 and The subnet mask is 255.255.240.0
In this case, you have assigned a class B address of 131.56.0.0 and selected subnet mask 255.255.240.0 to divide class B network y into different subnets. You want to determine Set the address range for the workstation n subnet 131.56.32.0.
First, pass the mask value to the binary form. Determine the rightmost bit 1 and the decimal value corresponding to the bit dd You will get progressive value 16.
The first device address will be 131.56.32.1. The next subnet address can be calculated by adding the progressive value to the subnet portion of the address. In this example, the next subnet will be 131.56.48.0. Subtracting 2 from this value will be the final value 131.56.47.254 within the address range of subnet 131.56.32.0.
