Hello Brano, Thank you. I hope I have understood how this must be done now. In case this may assist anyone else , this is how I assume we have to proceed: What is the CIDR equivalent for 255.255.255.224? bits 8 7 6 5 4 3 2 1 binary 1 1 1 1 1 1 1 1 value 128 64 32 16 8 4 2 1 255 binary 1 1 1 0 0 0 0 0 value 128 64 32 0 0 0 0 0 224 To answer the above question in an examination it is necessary to learn the Value figures shown above, and then use them as follows: An Octet of 255 value, always amounts to 8 bits. So of our example subnet mask 255.255.255.224 We know that 255.255.255 converts to: 8 + 8 + 8 = 24 (24 bits). This leaves (in this example) an Octet containing 224 value, requiring conversion as follows: From your Table, keep adding the highest Values to each other (left to right). 128 + 64 + 32 = 224. The number of Value figures you needed to add together to reach 224, was 3 (bits). Now add these 3 bits to those of the rest of the subnet mask address (which was 24), to reach your CIDR total, thus: 24 + 3 = 27 255.255.255.224 = 8 + 8 + 8 + 3 = 27 CIDR = /27

224 is equivalent to 128+64+32= 2^7+2^6+2^5 and 11100000 in binary. 19 is number of ones from left to right (8+8+3) that mask first 19 digits of IP address and the remnant digits are not taken into count because of multiplication by zeros. More on this subject see http://tools.ietf.org/html/rfc4632 Regards Milos

Hello, My MCTS book explains about "CIDR" ("Classless Inter-Domain Routing"), thus: For example "/24". The "CIDR" form shows the number of "bits" turned on in the "Subnet Mask": 255.255.224.0 is: 11111111.11111111.11100000.00000000, equalling 19 bits turned on, or "/19". Unfortunately the book does not explain how you do the calculation. I understand that 255 adds up to 8 turned-on bits. However, what is the calculation that I need to know in order to convert that 224 figure into the 3 turned-on bits? Robert339 Ps. I have tried inserting carrier returns to make spaces between the paragraphs in this message, but they are not saved.

Hello Milos, Thank you for the information. You say the "224" is equivalent to 128+64+32, but I am afraid I am missing something here. How did you calculate those figures? What I am looking for is the method for answering the following example question in an examination: What is the CIDR equivalent for 255.255.255.224? (MCTS 70:680 Assessment Test - Windows Configuration Study Guide by William Panek.) Regards, Robert339

Everyone has different way of thinking but for exam purposes a table is a good way to go. Just keep adding from left to right and ask a question if the current sum is bigger then 224 if it is then write down 0 and if it is not write down 1. bits 8 7 6 5 4 3 2 1 value 128 64 32 16 8 4 2 1 255 binary 1 1 1 0 0 0 0 0 128 64 32 0 0 0 0 0 224