Keep adding the magic number to obtain the next network.ģ0.0.100.80 Calculate the Total Available Subnets.Therefore the first network is 30.0.100.0.Our answer is calculated via the following steps: Here is a quick reference table for help when subnetting. We will now use the magic number as part of our examples. The magic number is simply the decimal representation of the host bits (in red). Boom!īefore we begin, it is worth mentioning that I will use a technique that uses something known as the magic number. I won't go into full-blown explanations I will cut straight past the BS, to the calculations. In order to show how subnetting is performed, I will step through a number of examples. We then use these subnet bits to calculate the number of additional subnets available.įigure 2 - Subnetting. Subnetting works by taking bits from the host part of an IP address in order to create subnet identifier bits (shown below). If you recall, we previously spoke about an IP address having a network and host identifier. Subnetting provides us with a number of benefits, such as: Subnetting OverviewĪ subnetwork or subnet is a logical subdivision of an IP network.The practice of dividing a network into two or more networks is called subnetting. VLSM is the "subnetting of subnets," which means that VLSM allows network engineers to divide an IP address space into a hierarchy of subnets of different sizes, making it possible to create subnets with very different host counts without wasting large numbers of addresses. This allowed networks to be aggregated, reducing the number of entries in the IP routing tables, whilst also allowing for greater efficiency in IP route processing. Classless networks do not use the subnet masks mandated by the different A, B or C classes, instead, the subnet mask (aka CIDR “bit” notation) is represented - ranging from /8 to /32. Classless Inter-Domain Routing (CIDR)ĬIDR (as per RFC1519) was implemented to allow the routing of both classful and classless networks. Both of which are key to modern day subnetting. The solution to this issue came in the form of - Classless Inter-Domain Routing (CIDR) and Variable Length Subnet Masking (VLSM). With the explosion of the internet and the need to address more and more devices, this issue was compounded and a solution was required. However, this was hugely inefficient, should a company require 2 IP addresses for a point to point link then the smallest classful network that could be assigned was a class C network, which would provide 256 hosts. Each class provided a different subnet mask based upon the IP range. This architecture provided three different network classes - known as class A, B and C. When the IP protocol was first released it was built upon a classful addressing architecture. For example, 255.255.0.0 would result in the first 2 octets (16bits) being the network bits and the remainder being the host bits. Within a subnet mask, the sequential bits that are on (1) are deemed the network bits and the bits that off (0) are considered the host bits. To determine which parts of an IP address are the network bits, and which are host bits, a subnet mask is used.
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