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07-18-2011, 10:33 PM #1New User
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- Jul 2011
Class B Private Reserved Address Space Question?
I purchased Todd's new 6th Edition CCNA Study Guide for the 640-802 Exam. I just finished chapter three and need help understanding Private Reserved Address Spaces.
I'd like to know if these address spaces can be calculated or if they're supposed to just be remembered?
I think I understand why the Class A Reserved Address Space is 10.0.0.0 - 10.255.255.255 and why Class C is 192.168.0.0 - 192.168.255.255 . However, the Class B Reserved Address Space of 172.16.0.0-172.31.255.255 doesn't make sense to me.
I've searched online for a method to calculate this hoping to understand how/why/where the 172.31 comes from but haven't had any success.
I'd greatly appreciate any help in understanding this. If I can understand it, it’ll help me understand how to arrive at the correct answer for #8 in the Written Lab 3.1: TCP/IP Section of the End of Chapter questions.
07-19-2011, 03:38 AM #2Moderator
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- Mar 2008
07-19-2011, 07:41 AM #3New User
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- Nov 2008
- Worcester, UK
If you look at the ranges of each private class in CIDR notation:
The mask goes up by 4 bits each time. As each class has a specific subnet mask (8, 16 and 24 repectively) this gives us one class A address - 10.0.0.0 /8, sixteen class B addresses - 172.16.0.0/16, 172.17.0.0 /16 .... 172.31.0.0 /16 and 256 class C addresses - 192.168.0.0 /24 ... 192.168.254.0 /24.
Compare the bit masks from the whole IP address space and the defaults for address class:
A: 8 - 8 = 2^0 = 1 address
B: 16 - 12 = 2^4 = 16 addresses
c: 24 - 16 = 2^8 = 256 addressesCCNP R&S, CCDP, CCNA DC
Currently studying: CCIE R&S
Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/
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