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Class B Subnetting

John OdenIn my video series Subnetting Demystified I attempted to create a learning tool that provides an overview of Internet Protocol version 4 or IPv4. It focuses on the topics that are most relevant to those who are studying for certification exams. We will be discussing some historical aspects of IPv4 as well as comparing it to where we are today.
The IPv4 address space has been allocated based upon the concept of classful addressing. This means that IPv4 addresses can be classified as Class A, B, or C by looking at the first few bits of an IPv4 adress. For more information about classful addressing, please see this video. Classful networking was able to provide an addressing model that has only three sizes: small-medium and large. This method of address allocation was inefficient as it was impossible to match the size and network size of each organization to which an address allocation was made.
The networking industry moved away from classful networking to a classless allocation model. This meant that the boundaries between subnetwork, network, and host parts of an IPv4 address could be anywhere along the 32-bit address length. This allowed for more precise allocations of IPv4 addresses as the allocation size could be more closely related to the size of an organization’s network. An organization with a small number of network-connected devices did not need to be assigned an entire Class C block.
Keep it classy
Why are we here to discuss Class B subnetting, you ask? Why is it that we continue to talk about address classes in the context subnetting? There are many reasons. One reason could be tradition. When we refer to an IPv4 address class, we really mean a network prefix length. What we really mean when we refer to a Class B subnet, is that we have an IPv4 address allocation with 16-bit prefix. We will then use the remaining 16 bits for subnet bits or host bits.
An example
The video Class B Subnetting shows us being given an address allocation of We are asked to use subnetting techniques in order to create a subnetting scheme that will provide at most 50 subnets each that can host at least 350 hosts. You might be wondering, “Is this supposed to be a class B subnetting example?” Isn’t this a Class A (, network? What’s the deal with the /17 prefix, you ask? Aren’t Class-B networks supposed to be /16?
The private Class A network includes the allocation. also falls within the private Class A network. We take, and subnet the first bit from the third octet. This creates the subnets, and What we really mean by “Class B subnetting” is that subnetting takes place in the third of the octet, as opposed to Class A subnetting which takes place in the fourth.

You can see how multiple subnets were created from the allocation. The first subnet was The next was And so on. Our requirement was that at least 50 of these subnets could be accommodated. We used 6 subnet bits to determine that the maximum number allowed was 26, which is 64 subnets. We went through the process to determine the first seven subnet addresses, broadcast addresses, and first and last hosts in each. What if we wanted the value of the last network?
Let’s take a look at the Class B Subnetting whiteboard to find the answer. The bits that we use for subnetting will be highlighted in yellow. We know that the first subnet of subnetting is and the second is 10.224.13