Subnet Mask

Methods of Subnet Masking

The methods of subnet masking are:

1. Variable-Length Subnet Masking (VLSM): Allows for the use of different subnet mask lengths within the same network, resulting in efficient use of IP address space.
    
2. Prefix Length Notation: Represents subnet masks using CIDR notation (e.g., /24) to simplify and standardize subnetting.
    
3. Classless Inter-Domain Routing (CIDR): Enables flexible allocation of IP addresses, breaking away from traditional class-based subnetting and allowing for more efficient use of IP addresses.
    
4. Subnetting by Borrowing Bits: Involves borrowing bits from the host portion of an IP address to create subnets, dividing a larger network into smaller, manageable subnetworks.

Important formulas to determine the subnets

There are several important formulas to determine the subnets:

1. Number of Subnets 

Formula: 2ⁿ, where n is the number of borrowed bits.

For example, borrowing 3 bits (n=3) creates 2³=8 subnets.
 

2. Number of Hosts per Subnet:

Formula: 2⁽³²⁻ⁿ⁾−2, where n is the number of borrowed bits.

For example, borrowing 3 bits (n=3) provides 2⁽³²⁻³⁾−2=30 usable host addresses.
 

3. Subnet Mask Calculation:

Formula: 256−2^subnet bits

For example, for a /27 subnet, 256−2²⁷=224, so the subnet mask is 255.255.255.224.
 

4. Total Number of Hosts in a Network:

Formula: 2^{(32−subnet bits)}−2

For example, for a /26 subnet, 2^(32−26)−2=62 usable host addresses.
 

5. Subnet Address Calculation:

Formula: subnet_number×block size

For example, if subnetting with a /28 mask, the block size is 16, and the third subnet is 3×16=48.

What is Network Addressing?

Network addressing is the process of assigning unique identifiers to a network device. This device can be your computer, printer, router, or server. It is used to identify these devices on the network and enable communication between them. Without using network addressing, devices will not be able to communicate with each other.

Now, you might be thinking of how this can happen. Let us understand the main concept of the subnet mask in computer networking.

Subnet Mask in Computer Networks

In computer networks, each subnet has unique IPs. The IP address has two parts that are network part and the host part. The network part of an IP address determines the network to which a subnet belongs. The host part of an IP address determines the specific subnets within that network. These both parts have four 8-bit octets. Here the subnet comes into the picture. 

A subnet mask is a 32-bit number. This number is used in determining the network and the host portion of an IP address. This 32-bit number is made by putting 1s at the network portion and 0s at the host portion. 

Now might be thinking, why use a subnet mask in computer networks? Let us discuss this.

Why use Subnet Mask?

The subnet mask is used for many reasons in computer networks. It helps network administrators to divide huge and complex networks into small subnetworks. There are various reasons to use a subnet mask:

• It helps to improve the network management. As we discussed that it divides the huge network, which helps to manage the traffic easily of small subnetworks.
   
• It helps to improve the security of the network. By dividing the network, network administrators can easily create small broadcast domains, which can help in preventing network attacks. 
   
• It helps in improving the scalability of the network. When we have a huge network, it is difficult to manage and scale that network. Due to this, we can easily scale and manage the network.

What is a Subnet Mask Calculator?

There are a number of ways to calculate the subnet mask but many of use subnet mask calculators. It is a tool that helps to calculate the subnet mask. It can also help us to find the subnet address range for a given IP address and the subnet mask prefix length. There are several varieties of subnet mask calculators available:

• Online subnet mask calculators
   
• Standalone subnet mask calculators
   
• Command-line subnet mask calculators
   
• Network scanner tools with subnet mask calculator
   

Now, let us understand how to calculate a subnet mask from an IP address.

Calculating Subnet Mask from an IP Address

Calculating the subnet mask from an IP address is the crucial part. To do this, we need to identify the network ID and host ID portions of the address. Then we need to determine the number of bits that are used in each of the portions. After getting the number of bits used for the network ID, we can construct the subnet mask. We have to set those network bits to 1 and set the remaining bits to 0.

There are various classes for IP addresses:

• Class A: This class’s IP address starts with 1 to 127. In this class, the network portion is of 8-bits, i.e., 11111111.00000000.00000000.00000000
   
• Class B: This class’s IP address starts with 128 to 191. In this class, the network portion is of 16-bits, i.e., 11111111.11111111.00000000.00000000
   
• Class C: This class’s IP address starts with 192 to 223. In this class, the network portion is of 16-bits, i.e., 11111111.11111111.11111111.00000000
   

There are some steps to calculate the subnet mask from an IP address:

Step 1: First, we need to write the IP address in the binary format. As we discussed that an address has four octets. So, we need to convert each octet of the IP address into its binary equivalent. 
 

Step 2: Then, we have to identify the network ID and host ID portions of the IP address. This is determined by the subnet mask that is being used. 
 

Step 3: Now, we have to count the number of bits that are used for the network ID portion. 
 

Step 4: Now, we have to construct the subnet mask. We can do this by setting the network ID bits to 1s and the host ID bits to 0s. 
 

By following these steps, we will get a subnet mask. This subnet mask will be used to identify the network and host portions of any IP addresses that belong to the same subnet.

Let us understand these steps with the help of an example.

Example to Calculate Subnet Mask

Suppose we have an IP address 180.166.1.0 and a subnet of 255.255.0.0. To calculate the subnet mask of this IP, you can follow these steps:

Step 1: Converting this IP to binary format.

So, our IP address is 180.166.1.0, and this IP address is class B, and we know that class B’s network portion has 16 bits with 1s.

By looking at the octet chart, we can convert this IP to binary format.

So, let us break these octets from our IP address and find a binary equivalent for each one of them.

180= 1+0+1+1+0+1+0+0
166= 1+0+1+0+0+1+1+0
1= 0+0+0+0+0+0+0+1
0= 0+0+0+0+0+0+0+0

So, after merging them, we will see the binary equivalent of the IP address.

10110100.10100110.00000001.00000000 is our IP’s binary equivalent.

 

Step 2: Now, we have to find the network and host ID portions of our IP.

So, 

Step 3: Now, we have to count the number of bits in the network portion.

So, the number of bits is 16.

 

Step 4: Now, we have to replace network ID bits with 1s and host ID bits with 0s.

So, we will get 11111111.11111111.00000000.00000000

So its decimal representation is 255.255.0.0
 

This subnet mask we have used to identify the network and host portions of the IP address that belong to the same subnet as 180.166.1.0.

Also see, Basic Networking Commands  and Subnetting in Computer Networks

Frequently Asked Questions

What is 255.255 255.0 in subnet mask?

255.255.255.0 is a subnet mask that signifies a Class C network with 24 bits allocated for the network and 8 bits for hosts.

Is 255.255 255.0 or 24?

Yes, 255.255.255.0 is equivalent to /24, indicating a subnet with 24 bits allocated for the network and 8 bits for hosts.

Is 255.255 255.255 IP address valid?

Yes, 255.255.255.255 is a valid broadcast address used to broadcast to all devices on the local network segment.

Is 10.0 255.255 8 a valid host address?

No, 10.0.255.255/8 represents a network address. In a /8 subnet, the last three octets are reserved for hosts, making 10.0.255.255 invalid for individual hosts.

Conclusion

In this article, we have discussed the concept of subnet mask. We have also discussed an example based on this concept. We have also discussed its advantages. You can check out our other blogs to enhance your knowledge:

• Introduction to subnetting
   
• Network Addressing
   
• IPV4 vs IPV6
   
• Digital Subscriber Line
   

We hope this blog helped you to understand the subnet mask. You can refer to our guided paths on the Coding Ninjas Studio platform. You can check our course to learn more about DSA, DBMS, Competitive Programming, Python, Java, JavaScript, etc. 

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