Question on calculating network ID from IP Adress and Subnet mask?

Question

I'm reading on basic networking (newbie). Below is an excerpt of the book, or see www.geocities.com/vugh/question.txt

"For example, here’s how the network address is extracted from an IP address using the 20-bit subnet mask from the previous example:
144 . 28 . 16 . 17
IP address: 10010000 00011100 00100000 00001001
Subnet mask: 11111111 11111111 11110000 00000000
Network ID: 10010000 00011100 00100000 00000000
144 . 28 . 16 . 0 "

QUESTION: I'm confused why the third octet of the network ID (and IP Address) is 16. Isn't it supposed to be 32? Also the last octet of the IP Address, isn't it supposed to be 9 instead of 17?

Answer 1

Not for that network. It's network address is 144.28.16.0. The broadcast address is 144.28.31.255. The useable IP addresses in the range would therefore be 144.28.16.1 - 144.28.31.254

(You got the binary for the third octet wrong. It should be 00010000, not 00100000.)

The next network with 20 bits would be 144.28.32.0 and the next after that would be 144.28.48.0 and so forth.

The final octet can be anything, since it's a host address (except for the network and broadcast addresses, of course.)

Answer 2

It should be 32 only not 16.as the third octet of network ID 00100000 stands for 32 in decimal.

Answer 3

Dear Obai

Your ip address is Class B range( 16 bit network bit), but your
subnet mask is 20 bits (ie., you assigned 4 host bits as network bit)


00010000
00100000
00110000
01000000
...

...
...
11100000

144.28.16.0 is the first combination of the network id for your IP address with 20 bits subnet mask

For this network id , First Host id is 144.28.16.1
For this network id, Last Host id is 144.28.31.254

144.28.32.0 is the second combination of the network id for your IP address with 20 bit subnet mask

144.28.48.0. is the third combination of the network id for your ip address with 20 bit subnet mask

144.28.64.0 is the fourth combination

......

....

144.28. 224.0 is the last combination

For this last combinations first host host id is

144.28.224.1

For this last combiantionslast host id is

144.28.239.254



Regarding this if you have any doubt mail me personnally


gazy...