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subnetting

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

subnetting

Postby Guest » Sun Jan 09, 2011 2:10 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
 

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Re:subnetting

Postby Guest » Sun Jan 09, 2011 2:12 pm

Its very simple

 

1) First expand the prefix length to subnet mask.

/20 expands to 255.255.240.0

2) 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 256

0

.

.

.

16

.

.

.

32

.

.

.

48

.

.

.

64

.

.

.

80

.

.

.

96

.

.

.

(so on) till 256

 

Now the valid IP ranges are as follows

 

172.29.80.1<- First valid IP

172.29.80.2

.

.

.

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

Postby Guest » Sun Jan 09, 2011 2:34 pm

Aha, Thank you.

Guest
 

Re:subnetting

Postby Guest » Sun Jan 09, 2011 3:07 pm

You welcome.

 

Thanks for rating

Guest
 

Re:subnetting

Postby Guest » Sun Jan 09, 2011 3:26 pm

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

Guest
 



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