Subnetting Question, Please Help!

Styles · 18 May 2009 at 21:06

Hi there.

Right I've been given an example question for a test I'm going to be taking at some point in the near future. I'm in telephony so the network side of things isn't as clear as it should be at the moment so I'm going to need some help with this one!

~The Question:

An SIP proxy has been configured with the IP address 172.80.54.23. The subnet mask is losed on the device as 172.80.54.23/20. Which is the correct number for this network:

a. 172.42.43.0
b. 172.43.64.18
c.172.80.48.0
d. 172.80.64.0

I know the answer is C but how do I get to that answer? I don't know how to work out how I get to the network address from the proxy IP address and net mask?

Any help would be really appreciated.

Thank you

garylouden · 18 May 2009 at 21:31

This is off the top of my head as i forgot a lot of this but this seems logical.

10101100.01010000.00110110.00010111

If you take the first 20 bits then you will get

10101100.01010000.00110000.00000000

Which will come out as 172.80.48.0

Peace · 19 May 2009 at 00:36

compare the binary representation of the pertinent octet of the subnet mask with the binary representation of the corresponding octet in the IP address.

172.80.54.23 = 00110110 (54)
255.255.240.0 = 11110000 (240)

we are only interested in the bits where there is a one in both octets;

00110110
11110000
=
32 +16 = 48

so your host is on subnet 172.80.48.0

Dist · 19 May 2009 at 01:07

As has allready been said, simply look at the binary notation. If your mask is /20, write down the first 20 digets of the IP in binary, and set the rest to 0, thats your network address. If you set the remaining digets to 1 instead of 0 then that gives you the broadcast address for that network.