That is as specific as you can get it. if it was 32 - 37 or even 32 - 39 then you could do 172.16.32.0 /21 But the 31 is your problem because you cross the bit boundary of 31 to 32.
So as shown below the common bits between 31-37 are 128 and 64 thus /18 is the mask. Regards, Tyson Scott - CCIE #13513 R&S, Security, and SP Managing Partner / Sr. Instructor - IPexpert, Inc. Mailto: [email protected] Telephone: +1.810.326.1444, ext. 208 Live Assistance, Please visit: www.ipexpert.com/chat eFax: +1.810.454.0130 IPexpert is a premier provider of Self-Study Workbooks, Video on Demand, Audio Tools, Online Hardware Rental and Classroom Training for the Cisco CCIE (R&S, Voice, Security & Service Provider) certification(s) with training locations throughout the United States, Europe, South Asia and Australia. Be sure to visit our online communities at www.ipexpert.com/communities and our public website at www.ipexpert.com -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Horongo MCPherson Tuahuku Sent: Wednesday, January 05, 2011 11:50 AM To: [email protected] Subject: [OSL | CCIE_RS] Binary and ACL I thought I mastered binary that I could even solve them by looking at it. How wrong! Personally I had no problems with binary maths and every time I saw it on my CCNA or CCNP exam I knew that they are my soft points. It seems the way the instructor does binary tricks me somehow and I happen not to follow. For example, I dont understand why he would break 7 contiguous subnets into three or more options when summarising them? Lets take 172.16.31.0 172.16.37.0 subnets. My understanding is that you convert all to binary and look at the pattern ( common bits) and then come up with summary address. Since the 1st two octets are similar, we should concentrate on the third octet. 128 64 32 16 8 4 2 1 172.16.31.0 0 0 0 1 1 1 1 1 172.16.32.0 0 0 1 0 0 0 0 0 172.16.33.0 0 0 1 0 0 0 0 1 172.16.34.0 0 0 1 0 0 0 1 0 172.16.35.0 0 0 1 0 0 0 1 1 172.16.36.0 0 0 1 0 0 1 0 0 172.16.37.0 0 0 1 0 0 1 0 1 Summary 172.16.0.0 /18? Hope to hear from you all. Regards, Horongo _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com
