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What subnet mask would be associated with the IPv4 prefix of /28?

• 255.255.255.252
• 255.255.255.240
• 255.255.255.224
• 255.255.255.0
• 255.255.255.248

The correct answer is 255.255.255.240.

Understanding IPv4 Subnet Masks and Prefix Notation

To fully grasp the relationship between the IPv4 prefix notation (such as /28) and the associated subnet mask (255.255.255.240), we need to first review the basic concepts of IPv4 addressing and subnetting.

IPv4 addresses are 32-bit binary numbers, usually expressed in decimal form as four octets separated by dots (e.g., 192.168.1.1). Each octet represents 8 bits, and each bit can be either a 0 or a 1. This 32-bit structure allows for approximately 4.3 billion unique addresses (2^32). However, due to the structure of IP address classes, reserved addresses, and the need for private network addresses, not all of these are usable for general public networks.

A subnet mask is used in conjunction with an IPv4 address to divide the network into smaller sub-networks, or subnets. The subnet mask separates the IP address into two parts:

• Network portion: Defines which part of the address is the network identifier. This part is common to all devices on the same subnet.
• Host portion: Defines which part of the address identifies individual devices (hosts) within the subnet.

The subnet mask is also a 32-bit number that is typically represented in the same dotted-decimal format as the IP address. In a subnet mask, the bits that represent the network portion are set to 1, and the bits that represent the host portion are set to 0. The more bits set to 1 in the subnet mask, the smaller the number of hosts that can exist within the subnet.

CIDR Notation (Prefix Notation)

Classless Inter-Domain Routing (CIDR) notation, or prefix notation, provides a shorthand way to represent the subnet mask. Instead of writing out the full subnet mask in dotted-decimal form (like 255.255.255.240), we can express it as a prefix length. For example, /28 means that the first 28 bits of the subnet mask are set to 1, representing the network portion, and the remaining 4 bits are set to 0, representing the host portion.

Converting Prefix Notation (/28) to Subnet Mask

Let’s break down the process of converting the /28 prefix into the corresponding subnet mask.

1. Binary Representation

First, we need to understand that a subnet mask is a 32-bit binary number. In CIDR notation, the number after the slash (e.g., /28) tells us how many of the 32 bits are set to 1. So, for a /28 prefix, the first 28 bits are set to 1, and the remaining 4 bits are set to 0.

The 32-bit subnet mask for /28 looks like this in binary:

11111111.11111111.11111111.11110000

Let’s break it down:

• The first three octets (24 bits) are all 1s: 11111111.11111111.11111111.
• In the fourth octet, the first 4 bits are 1s and the last 4 bits are 0s: 11110000.

2. Converting Binary to Decimal

Next, we convert each octet from binary to decimal to obtain the dotted-decimal format of the subnet mask.

• The first octet, 11111111, is equal to 255 in decimal.
• The second octet, 11111111, is equal to 255 in decimal.
• The third octet, 11111111, is equal to 255 in decimal.
• The fourth octet, 11110000, is equal to 240 in decimal.

Therefore, the subnet mask for the /28 prefix is:

255.255.255.240

Breakdown of Subnet Mask 255.255.255.240

Now that we know that /28 corresponds to a subnet mask of 255.255.255.240, let’s break down what this subnet mask actually means in terms of networking.

1. Number of Hosts Per Subnet

The number of bits allocated to the host portion (the bits set to 0 in the subnet mask) determines how many unique IP addresses can be assigned to devices in the subnet. In this case, the last 4 bits of the 32-bit address are set to 0, which means 4 bits are available for hosts.

The number of hosts is calculated using the formula:

Number of Hosts = 2^n - 2

Where n is the number of host bits, and we subtract 2 because one address is reserved for the network address and one for the broadcast address.

For /28, the number of host bits is 4, so:

Number of Hosts = 2^4 - 2 = 16 - 2 = 14

This means that a subnet with a /28 prefix can support up to 14 usable IP addresses for devices (hosts).

2. Number of Subnets

The number of subnets that can be created depends on how many bits are borrowed from the host portion to create the subnet portion. With a /28 prefix, we are using 28 bits for the network portion and 4 bits for the host portion. This allows us to create multiple subnets within a larger network.

For example, if the network was originally configured with a /24 subnet mask (255.255.255.0), using a /28 subnet mask allows us to divide the /24 network into smaller /28 subnets. The number of /28 subnets that can be created from a /24 network is calculated as:

Number of Subnets = 2^(new subnet bits)

In this case, we borrowed 4 bits from the host portion (since /24 uses 24 bits for the network portion and /28 uses 28 bits), so:

Number of Subnets = 2^4 = 16

This means that a single /24 network can be divided into 16 smaller /28 subnets.

3. Subnet Range and Network Addressing

Let’s take an example of how the IP addressing works within a /28 subnet. Assume we have a network address of 192.168.1.0/28. The address range for this subnet would be:

• Network address: 192.168.1.0 (the first address in the subnet, reserved for identifying the subnet itself)
• First usable IP address: 192.168.1.1 (the first assignable address for hosts)
• Last usable IP address: 192.168.1.14 (the last assignable address for hosts)
• Broadcast address: 192.168.1.15 (the last address in the subnet, reserved for broadcast communications)

So, the IP address range for a /28 subnet is from 192.168.1.1 to 192.168.1.14, with a total of 14 usable IP addresses.

Benefits of a /28 Subnet Mask

1. Efficient Use of Address Space: The /28 subnet mask allows network administrators to efficiently divide large networks into smaller subnets, making it possible to allocate only as many IP addresses as needed for each subnet. This prevents wastage of IP addresses, which is particularly important in IPv4 networks where address space is limited.
2. Improved Network Performance: By dividing a network into smaller subnets, you can reduce network congestion. Broadcasts are confined to individual subnets, rather than propagating across the entire network, which can improve performance, especially in larger networks.
3. Security: Segmenting a network into smaller subnets can help isolate different parts of the network for security purposes. For example, you can assign different security policies to different subnets, helping to protect sensitive data or prevent unauthorized access.
4. Simpler Network Management: Subnetting a network with a /28 prefix makes it easier to manage and troubleshoot. Each subnet can be assigned to a specific department or function within an organization, simplifying the administration of IP addresses and the enforcement of network policies.

Conclusion

A /28 prefix corresponds to the subnet mask 255.255.255.240, which provides 14 usable IP addresses in each subnet. This subnet mask is particularly useful when network administrators need to efficiently allocate address space and create smaller subnets. Understanding how prefix notation translates to subnet masks, and how subnet masks are used in network addressing, is crucial for designing and managing efficient IPv4 networks.