ip address

if netmask 255.255.255.224 is used, which ones below are valid for hosts?
A. 201.45.116.159
B. 134.178.18.62
C. 192.168.16.91
D. 92.11.178.93
E. 217.63.12. 24
F. 15.234.118.63
i would say B,C,D, and E.
do i need to look at each one based on its class and exclude the network and broadcast addresses?

You can't look at it based on class, this is a classless IP address -- its a /27.. what you have to do is find out what block of addresses for each subnet can be used.. since we are subnetting in the 4th octet, we'll do this

256 - 224 = 32 is our block size..

this means that all the subnets will be in multiples of 32.. first subnet will start at 32, second at 64, 96, 128 etc..

All the broadcast addresses will be one less than the next subnet, so the first subnet broadcast will be 63, then 95, then 127 etc etc ...

So now you can tell which addresses are in a valid range (the last octet should fall between two multiples of 32, and not be one less than the higher multpile as this would be the broadcast address).

So the first one :

.159 -- lies in the .128 subnet, but it is just one less than the next subnet address which is 160, so this is a broadcast address for the .128 subnet.

.62 -- This is valid.. it falls in the .32 subnet, and is less than the broadcast address of .63

.91 -- Also valid, falls in the .64 subnet, who's broadcast address is .95 (one less than the next subnet which is .96).

.93 -- Also valid for the same reasons as above

.24 -- Hmm here's a tricky one.. if subnet zero is allowed, then this is valid, it will fall in the range 0 - 31 (32 being the next subnet).. however on Cisco exams, subnet zero is not allowed (unless they changed it).

.63 -- This is not valid, its the broadcast address for the .32 subnet, since .64 is the next subnet and it is just one less than 64.


You are right, ignore the network address (what I call the subnet address) and the broadcast address (1 less than next subnet), but you have to ensure that it falls in the subnet ranges this mask generates.


Damn I miss this stuff

.24 -- Hmm here's a tricky one.. if subnet zero is allowed, then this is valid, it will fall in the range 0 - 31 (32 being the next subnet).. however on Cisco exams, subnet zero is not allowed (unless they changed it).

I'm attending CCNA2... in a cisco's test that ip address is not valid because the "subnet zero" is not raccomended to be use

Yep, they dont allow subnet zero on the exams..

be warned though, all cisco equipment comes with 'subnet zero' enabled by default.. i will never understand this...

Yep, they dont allow subnet zero on the exams..

be warned though, all cisco equipment comes with 'subnet zero' enabled by default.. i will never understand this...

hehehe i know

my cisco's teacher told me that "subnet zero" can be a problem only with old device

thank you so much sahirh.
all the best.
mixaogion