Subnetting Question

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hi all

i thought i was ok at subnetting til i got a question simillar to this:

which addresses are valid host addresses given the subnet mask 255.255.255.248

192.168.200.87
194.10.10.84
223.168.210.100
200.100.100.154
200.152.2.160
196.123.142.190

choose 3

if someone could should me how to work this out i'd be grateful

cheers

 

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Hm - I'd say that 4 of them are valid and 2 invalid.

Method - use the mask to find the *machine address* (i.e. the part where the mask is zero). If the machine address is all zeros or all ones then it isn't a valid *host* address.

Of course - I may not have spotted some nuance....


Harry.

 

Quick method:

256 - 248 gives 'magic number' of 8.

Therefore every subnet network address will be a multiple of 8, starting with 0 :
0, 8, 16, 24, 32....etc

Every broadcast address will be one less than a possible subnet network address :
7, 15, 23, 31......etc

Any value in the fourth octet that matches one of these numbers can not be a host address.

Easy really....

 

hi buddy

thanx, easy when u know how

ok wot about this one:

10.149.77.147 /12

wot is the broadcast address

10.255.255.255
10.144.255.255
10.149.255.255
10.149.191.255
10.159.255.255

cheers

 

Why don't you give it a shot with the information you have rather than ask someone to give you the answer?

 

not lookin for the answer buddy. just looking for a way to work it out. the /12 is confusing me.

am i rite in saying that the magic number here is 16 so 0, 16,32,48,64

after that i get lost

cheers

 

The /12 indicates how many '1' bits there are in the mask. (Counting from the 'left' - i.e. the most significant position).

So calculate from there.

Harry.

 

ah the magic number is 16 so the network will be 144, so the next network is 160 - 1 = 159

answer 10.159.255.255

 

Just wanted to see how far you could work it out on your own, that's all. Then we can give you a nudge in the right direction, as Harry has graciously done.

And... unless my calculations are off, you have answered correctly with 10.159.255.255. ;)

 

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