subnet mask puzzle
Hi,
considering the following scenario:
"You are the administrator of your company's network, which uses TCP/IP as its only protocol. The network uses the private address space of 172.16.0.0/16.
Your company projects rapid growth during the next three years. To reduce the risk of network saturation, you decide to subnet the network. You will eventually need to accommodate a maximum of 17 subnets. Each subnet will include a maximum of 1,200 hosts. Which subnet mask should you configure to meet both your current needs and your projected needs?"
A. 255.255.224.0
B. 255.255.240.0
C. 255.255.248.0
D. 255.255.254.0
E. 255.255.255.0
my questions are:
1) doesn't "172.16.0.0/16" mean IP address of 172.16.0.0 with a subnet mask of 255.255.0.0? why do we have to configure another subnet mask?
2) 17 subnets with each subnet including 1200 hosts will be 20400 hosts. so 2^15 is 32768 which is larger than 20400. so i thought i should leave 15 bits for these hosts and therefore 17 bits is left(32-15=17). so in binary, that would be 11111111.11111111.10000000
Thanks a lot in advance,
KEN
considering the following scenario:
"You are the administrator of your company's network, which uses TCP/IP as its only protocol. The network uses the private address space of 172.16.0.0/16.
Your company projects rapid growth during the next three years. To reduce the risk of network saturation, you decide to subnet the network. You will eventually need to accommodate a maximum of 17 subnets. Each subnet will include a maximum of 1,200 hosts. Which subnet mask should you configure to meet both your current needs and your projected needs?"
A. 255.255.224.0
B. 255.255.240.0
C. 255.255.248.0
D. 255.255.254.0
E. 255.255.255.0
my questions are:
1) doesn't "172.16.0.0/16" mean IP address of 172.16.0.0 with a subnet mask of 255.255.0.0? why do we have to configure another subnet mask?
2) 17 subnets with each subnet including 1200 hosts will be 20400 hosts. so 2^15 is 32768 which is larger than 20400. so i thought i should leave 15 bits for these hosts and therefore 17 bits is left(32-15=17). so in binary, that would be 11111111.11111111.10000000
Thanks a lot in advance,
KEN
I don't disagree with the existing comment but prefer another "slant" on this...
1) Yes, the specification of "172.16.0.0/16" means that the address range for this subject spans 16-bits of IP space or 172.16.0.0 -> 172.16.255.255. It's quite common for routers and other network hardware to specify their address ranges in this convenient format. It means, however, that you have 2^16 or 65536 possible IPs at your disposal.
2) You need to allocate IPs so that at least 1200 hosts can be on each subnet. Unfortunately, 1200 is not a power of two so you'll need to "round up" to the next power of two. 2^11 is 2048 which is the first power of two that is large enough to accomodate your subnet requirement of 1200 hosts/subnet. That will consume 11 out of the 16 bits of your IP space leaving you with 5 bits or 2^5 = 32 subnets. Since you only needed 17, that gives you a fit where you can have up to 32 subnets of 2048 hosts each.
So that leaves us with a netmask for each subnet of:
11111111111111111111100000
1) Yes, the specification of "172.16.0.0/16" means that the address range for this subject spans 16-bits of IP space or 172.16.0.0 -> 172.16.255.255. It's quite common for routers and other network hardware to specify their address ranges in this convenient format. It means, however, that you have 2^16 or 65536 possible IPs at your disposal.
2) You need to allocate IPs so that at least 1200 hosts can be on each subnet. Unfortunately, 1200 is not a power of two so you'll need to "round up" to the next power of two. 2^11 is 2048 which is the first power of two that is large enough to accomodate your subnet requirement of 1200 hosts/subnet. That will consume 11 out of the 16 bits of your IP space leaving you with 5 bits or 2^5 = 32 subnets. Since you only needed 17, that gives you a fit where you can have up to 32 subnets of 2048 hosts each.
So that leaves us with a netmask for each subnet of:
11111111111111111111100000
ASKER
sorry maybe i didn't put it clearly.
regarding my first question: i knew it's a private IP range. i just don't understand why i was asked to configure another subnet mask even though i had been given "/16", which is the same as 255.255.0.0......i mean it seems to me that there would be a clash between this 255.255.0.0 and 255.255.248.0
Thanks
KEN
regarding my first question: i knew it's a private IP range. i just don't understand why i was asked to configure another subnet mask even though i had been given "/16", which is the same as 255.255.0.0......i mean it seems to me that there would be a clash between this 255.255.0.0 and 255.255.248.0
Thanks
KEN
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ASKER
Thanks for your help!
see you next time,
KEN
see you next time,
KEN
2) You need 17 different subnets, so you need to reserve at least 5 bits to differentiate between the subnets--this will still leave 2^11 (2048) possible hosts for each subnet. What you did was calculate what subnet mask you'd need to put every single machine onto a single subnet, which isn't what the question asked.