subnetting

CCNA, CCDA, CCNP, CCDP, CCIP, CCVP and CCIE
Guest

subnetting

Post by Guest » Sun Jan 09, 2011 3:10 pm

I am comfortable in subnetting class C IP addresses.  How would you solve the following?What is the last valid host on the subnetwork 172.29.80.0/20? Thanks.

Guest

Re:subnetting

Post by Guest » Sun Jan 09, 2011 3:12 pm

Its very simple 1) First expand the prefix length to subnet mask. /20 expands to 255.255.240.02) Any octet that contains value other then 0 or 255 is called interesting octet. In our case 3rd octet contains 240 so do the following  subtract interesting-octet value (240) from 256.  256 - 240 = 16. Now start from 0 and add 16 till 2560...16...32...48...64...80...96...(so on) till 256 Now the valid IP ranges are as follows 172.29.80.1<- First valid IP172.29.80.2...172.29.95.254 <- Last valid IP Now how did we get this ? Remember, the last valid IP is Just BEFORE the broadcast address of that subnet. In our case the subnet is 172.29.80.0/20, the broadcast address of this subnet is all ones in host portion. So the value will be 172.29.95.255 for broadcast address. Just minus 1 from the last octet and you get the last valid IP. I have assumed you know how to do subnetting in binary.

Guest

Re:subnetting

Post by Guest » Sun Jan 09, 2011 3:34 pm

Aha, Thank you.

Guest

Re:subnetting

Post by Guest » Sun Jan 09, 2011 4:07 pm

You welcome. Thanks for rating

Guest

Re:subnetting

Post by Guest » Sun Jan 09, 2011 4:26 pm

You have a class B network with a /24 mask. How many usable subnets exist?

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