Daughterofrnbsoul wrote: » Hey guys i plan on taking the CCENT 100-105 very soon, and i understand that time is the essence for subnetting. Im pretty that I've gotten the concepts of subnetting down, and finding out what given ip address / mask belongs to which subnet using the Magic Hat, 256 - Mask = Value method. However, i am having trouble with speed, especially the bigger ip address that fall into a specific subnet like 172.16.166.1/21. Then just increment in the Interesting Octet (3rd)+8without going over the 166.My problem is doing the addition over and over which can take up a lot of time. 8+8+8+8+8+8+8+8+8+8+8+8+8+8+8+8+8+8+8= 160. Is their a special formula that i can use without just doing addition over and over and over ? To find my Subnet ID for a specific address ? V 128 192 224 240 248 252 254 255 128 64 32 16 8 4 2 1 8............
clarson wrote: » there are a couple of ways. 1) divide 166 by 8 that equals 20.something. drop the something, take 8x20 = 160 2) look at the lines of your data and do comparisons 128 192 224 240 248 252 254 255 128 64 32 16 8 4 2 1 second line, what are multiples of 8. 128 64 32 16 8 what is less than 166? biggest number is 128 128 is 8 x 16 so you have already did lots of additions. what is 128 + 64. from the first line that is 192. that is > 166 so it isn't going to work what is 128 + 32. that is 160. 166 - 160 is less than 8 So that is your answer. And, it only took one addition.
