![]() Once we know the size (magic number) of the resulting divisions of the original network range, finding the multiple of this number closest to the original IP octet value without going over gives us the desired resident subnet ID. Another important fact we can notice from the figure is that the size of the resulting subnets can be calculated by subtracting the interesting octet value from 256, which corresponds to the magic number concept used in the OCG book. This is because the interesting octet, by definition, will have a binary value of all 0’s in the host portion of the octet and all 1’s in the subnet portion. The figure highlights one important fact: in all subnetting schemes, the last subnet octet always corresponds to the interesting octet described in the OCG book. The next one divides the network in eight subnets (/27) and the last one in sixteen subnets (/28). The second one corresponds to a /26, which divides the network in four subnets. The first one corresponds to a /25 mask, which divides the original network in two subnets. The figure shows four different subnetting schemes. Therefore, the subnet ID will be 10.0.50.72/29.īut, at this point, you may ask yourself: why does it work? In order to understand this process, let’s have a look at the following figure: The value ‘X’ will be the largest multiple of 8 that is less than 76, which is 72. ![]() To find the last octet ‘X’, we have to follow the step 3 and calculate the magic number as 256 – 248 = 8. Following the previous procedure, since the first three octets are 255, to find the resulting subnet ID we just need to copy the first three octets of the IP address: 10.0.50.X. In this case, the subnet mask in dotted decimal notation (DDN) is 255.255.255.248. Calculate the subnet ID octet as the largest multiple of the magic number that is less than the correspoding IP octet.Īs an example, let’s find the resident subnet ID of the IP address 10.0.50.76/29.Calculate the magic number as 256 – V, where V is the decimal value of the interesting octet.If the mask octet is an interesting octet:.And now you have your subnet number, broadcast address, and host range without having to use any binary. If the mask octet is 0, put a 0 in the same position. To find the host range, simply add 1 to the final octet of the subnet number and subtract 1 from the final octet of the broadcast address.If the mask octet is 255, just copy the corresponding IP octet in the same position.The process of finding the resident subnet ID of an IP address described in the book can be summarized as follows. The OCG book calls that octet the interesting octet. Finding the subnet ID given an IP address and a non-trivial subnet mask can be a challenge at first, but with practice, it can become a second nature making possible to solve subnetting problems in just a few seconds. Since, by definition, a subnet mask cannot interleave 0s and 1s, the possible values for that octet are: 128, 192, 224, 240, 248, 252 and 254. Mastering subnetting is a fundamental skill in network engineering. A difficult mask is a subnet mask that has one octet that is neither 0 nor 255. In this book, the author describe a process to find the subnet ID when using a difficult mask. One of the main study resources I used for the CCNA was the Cisco Official Cert Guide Book (OCG). r/CompTIA holds no responsibility for certification nullification which may be caused by visiting links submitted to this subreddit.Īsking for, or posting links to Braindumps, Unauthorized reproduction of testing material, copyrighted material or other content that is in violation of the CompTIA Candidate Agreement will result in removal of post, up to being banned from this community.Mastering subnetting is a fundamental skill in network engineering. Note Please be advised that the use of some of the links above may violate CompTIA's certification policies.
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