Why Subnetting ? When there is a network with a size large, it may sound fine but it gets to a stage when that network performance starts to suffer due to too much traffic. One of the most effective solutions to the problem just mentioned here is breaking the network into smaller sizes by the technique of subnetting. What is subnetting? Subnetting is the technique of segmenting a TCP / IP network into manageable sizes that are smaller. Too much traffic on a single network would slow the network down. Therefore subnetting a large network is breaking it up into smaller separate yet interconnected network. This isolates most traffic to its originating subnet. This however does not stop communication between subnets as they are interconnected but will be limited to when the destination host is on a different subnet to the source / originating subnet. Firstly, there should be some sort of planning for each subnet in terms of size of the network, number of hosts required and number of subnets required. Subnets could also be planned based on various circumstances such as users within a department. We will discuss planning in details later. The purpose of subnetting is defeated if multiple subnets share a common hub. Before we proceed, we need to know our 2 to the power of n (n is the number of host bits). However, we will concern ourselves with only 1-8 i.e. We also need to remind ourselves that each octet has 8 bits each. Each bit is indicated in the prefix of the subnet mask. If for example we use 192 . 160 . 1 . 0/24 with default subnet mask 255 . 255 . 255 . 0, its prefix is /24 meaning that the first 24 bits (the first 3 octets) is the network portion. This subnet mask table will come in handy in our calculations. Formula to calculate number of usable hosts = 2 to the power of n – 2 (n is the number of host bits remaining after bits are borrowed). 2 is subtracted because 2 addresses must be reserved always for the Network address and Broadcast address respectively. If we borrow 1 bit from the host portion, we will create 2 subnets. Thus The subnets now have addresses as follows Are you wondering how I arrived at the new address and subnet mask ? Well ! How many bits did we borrow? 1 (8 host bits – 1 borrowed bit = 7 bits left for hosts) How many subnets were thus created? (2 to the power of 1) = 2 subnets How many usable hosts per subnet? (2 to the power of 7) - 2 = 126 i.e (128 – 2 = 126) What is the new subnet mask? 255 . 255 . 255 . 128 What is the block size? 256 – 128 = 128 => Block size = 256 – value of octet in subnet mask If we borrow 2 bits, we will create 4 subnets. The subnets now have addresses as follows How to work out the 4 subnets Are you wondering how I arrived at the new address and subnet mask too? Well ! How many bits did we borrow? 2 How many subnets were thus created? (2 to the power of 2) = 4 subnets How many usable hosts per subnet? (2 to the power of 6) - 2 = 62 i.e (64 – 2 = 62) What is the new subnet mask? 255 . 255 . 255 . 192 number of hosts required What is the block size? 256 – 192= 64 => =>Block size = 256 – value of octet in subnet mask If we borrow 3 bits, we will create 8 subnets. The subnets now have addresses as follows How to work out the 8 subnets Are you still wondering how I arrived at the new address and subnet mask too? Well !
How many bits did we borrow? 3 How many subnets were thus created? (2 to the power of 3) = 8 subnets How many usable hosts per subnet? (2 to the power of 5) - 2 = 30 i.e (32 – 2 = 30) What is the new subnet mask? 255 . 255 . 255 . 224 What is the block size? 256 – 224= 32 => Block size = 256 – value of octet in subnet mask References http://searchnetworking.techtarget.com/feature/Subnetting-to-increase-performance https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.htm |
CategoriesAuthor I have a passion for networking and use this medium for tutorials in Networking Archives
April 2017
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