subnetting

marorag

good day good ppl
i'm planning on takin the ccent exam in the next 2 weeks but want to concentrate mostly on subnetting coz this simply looks like the difficult part of the examination, so i have a situation underneath of which i need some explanation on how you get the answer..

your network has been assigned the network number 134.100.0.0.Your network manager
requests that you come up with a subnet mask that allows for atleast 500 subnets, but no subnet
must be able to hold more than 120 hosts.What is the best subnet mask to use?

derek_gruending

I've almost found a solution for you, problem is that you have 126 hosts per subnet. Basically, use this formula:

IP Address: 172.16.0.0 /25

The number of subnets = 2 ^ (subnet_mask_in_question - default_subnet_mask)
The number of hosts = (2 ^ (32 - subnet_mask_in_question)) - 2

So, basically its:

2 ^ (25 - 16) = 2 ^ 9 = 512 subnets
and
(2 ^ (32 - 25)) - 2 = 2 ^ (7 - 2) = 126 hosts

Hope this helps

notun1

marorag wrote: »

good day good ppl
i'm planning on takin the ccent exam in the next 2 weeks but want to concentrate mostly on subnetting coz this simply looks like the difficult part of the examination, so i have a situation underneath of which i need some explanation on how you get the answer..

your network has been assigned the network number 134.100.0.0.Your network manager
requests that you come up with a subnet mask that allows for atleast 500 subnets, but no subnet
must be able to hold more than 120 hosts.What is the best subnet mask to use?

As class B. you have 16 host bit. for subnetting you can take first 10 bit for network and 6 bit for host to get this criteria.

PhildoBaggins

[HTML] B B B B B B B B C C C C C C C C
INCR 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1
CIDR 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
MASK 128 192 224 240 248 252 254 255 128 192 224 240 248 252 254 255
HOSTS 32766 16382 8190 4094 2046 1022 510 254 126 62 30 14 6 2 0 0
Bsub 0 2 6 14 30 62 126 254 510 1022 2046 4094 8190 16382 0 0
Csub 1 1 1 1 1 1 1 1 0 2 6 14 30 62 0 0[/HTML]

Memorize this table and these questions will become much simpler to answer.

The answer is the first C column.

You will subnet your class B network with 25 network bits. 255.255.255.128 giving you 126 hosts max over 510 subnets (no ip zero) or 512 networks with ip zero.

i'm not sure why you would want no more than 120 hosts but if that was the deal breaker on the question you could move up one more network bit to a /26 CIDR and that would give you a mask of 255.255.255.192

allowing 62 hosts per subnet and 1022 networks or 1022+2 if ip zero subnet is on.

derek_gruending is pretty much on it I prefer memorizing the chart. It really helped during my ICND1 I used it probably 5 or 6 times.

notun1

you are not allowed to get more than 120 host. so basically 2^10 =1024 should be network and 2^6=64-2 should be HOst.

derek_gruending wrote: »

I've almost found a solution for you, problem is that you have 126 hosts per subnet. Basically, use this formula:

IP Address: 172.16.0.0 /25

The number of subnets = 2 ^ (subnet_mask_in_question - default_subnet_mask)
The number of hosts = (2 ^ (32 - subnet_mask_in_question)) - 2

So, basically its:

2 ^ (25 - 16) = 2 ^ 9 = 512 subnets
and
(2 ^ (32 - 25)) - 2 = 2 ^ (7 - 2) = 126 hosts

Hope this helps

wbosher

notun1 wrote: »

you are not allowed to get more than 120 host. so basically 2^10 =1024 should be network and 2^6=64-2 should be HOst.

I agree, the mask should be 255.255.255.192 or /26.