Subnetting: What Is a Subnet? How It Works

• Subnetting Definition

  Subnetting Definition

  Subnetting is the process of creating a subnetwork (also known as a subnet) within a network. Network interfaces and devices within a subnet can communicate with each other directly. Routers facilitate communication between different subnets.
• How to create a subnet

  How to create a subnet

  A subnet contains three main elements:

  1. Network address (or) subnet ID: This is always the first address of the subnet. 
  2. Broadcast address: This is always the last address of the subnet. A packet forwarded to the broadcast address is broadcasted to all the addresses in a subnet. 
  3. Subnet mask (or) netmask: This is the bitmask used to identify the subnet of an IP address by applying bitwise AND operation with the netmask and the IP address. All the IP addresses in a subnet contain an identical most-significant bit group. This is how a router identifies the subnet of an IP address since data packets only contain the destination IP address.

  Consider the following example:

  Say you have a network with IP addresses ranging from 192.168.255.0 to 192.168. 255.255 and want to create two subnets. 

  In general, each IP address consists of two parts: network identifier and host identifier. All the host addresses in the same network will have an identical network identifier but a unique host identifier. 

  In this case, the first 24 bits (i.e., 192.168.255) represent the network identifier, and the remaining 8 bits represent the host identifier.

  Considering x is the number of 1 bits needed in the host identifier part of the subnet mask, 2x is the number of subnets. As you need 2 subnets, x is 1.

  192.168.255.0 is also the common network identifier for the whole network, and its subnet mask is 255.255.255.0

  Let’s convert these two addresses into binary:

  192.168.255.0 = 11000000.10101000.11111111.00000000

  255.255.255.0 = 11111111.11111111.11111111.00000000

  As you need 2 subnets, one 1 bit is required in the host identifier part of the subnet mask. Hence, the subnet mask required is:

  11111111.11111111.11111111.10000000 = 255.255.255.128

  Accordingly, the subnet ID of the first subnet is 192.168.255.0, and the broadcast address is 192.168.255.127. Similarly, the subnet ID of the second subnet is 192.168.255.128, and the broadcast address is 192.168.255.255.

  Except for the subnet ID and broadcast address, you can use all the remaining IP addresses in a subnet as host addresses.

  Calculating a subnet mask can be time-consuming and confusing. Instead, you can use a subnet mask calculator to calculate a subnet mask accurately.
• What is a point-to-point subnet?

  What is a point-to-point subnet?

  A point-to-point subnet is a particular type of subnet used in point-to-point links that facilitate direct communication between two routers. This subnet consists of a 31-bit subnet mask, leaving only two possible addresses in the network. Since point-to-point subnets contain only one host, a different broadcast address isn’t necessary.
• What is IPv6 subnetting?

  What is IPv6 subnetting?

  IPv6 subnetting works slightly differently from IPv4 subnetting. Unlike IPv4 addresses, IPv6 addresses are 128-bit in length and include 16 designated bits for subnetting after the most significant 48 bits allocated to the network identifier. Hence, the most significant 64 bits represent the network identifier, and the least significant bits represent the host identifier.

  You can also borrow bits from the host identifier similar to IPv4 subnetting, but this can create problems with host address auto-configuration.
• Benefits of subnetting

  Benefits of subnetting

  Subnetting offers three common benefits:

  1. Improved Network Performance: When a device broadcasts a packet, it’ll reach all the network devices, burdening the network. Without proper context, broadcast packets can also spam devices within the network. This can lead to degraded network performance. By creating subnets, you can limit the scope of intranetwork broadcast messages to a specific subnet. This also enables efficient communication between devices in a subnet and sends a packet for routing outside the subnet if a destination address isn’t part of the subnet, leading to minimum network congestion.
  2. Enhanced Network Security: If an attacker gains unauthorized access to your network, all your network devices may be compromised. With subnets, you can limit a network breach by isolating the compromised subnetwork.
  3. Simplified Network Management: IPv4 host addresses are classified into three classes: Class A, Class B, and Class C.

  a.Each Class A block contains 16,777,214 addresses.

  b. Each Class B block contains 65,534 addresses.

  c. Each Class C block contains 254 addresses.

   

  If your organization requires more than 254 host addresses, then a Class B allocation would be necessary. Without subnetting, this may waste huge amounts of addresses.

  In addition, subnetting simplifies network monitoring and management by helping you logically group network devices.
• Limitations of subnetting

  Limitations of subnetting

  While subnetting offers many advantages, it can also result in some disadvantages:

  • Communication between one subnet to another subnet requires a router. A poorly configured or fatally failed router can significantly impact your organization’s network. 
  • Since each subnet requires dedicated IP addresses as subnet ID and broadcast address, it wastes IP addresses. 
  • Creating too many subnets can create unnecessary complexity and impact the effectiveness of network administration.