Subnetting Question.

anfearr

Hi,

A quick query which has me stumpted.

I take a subnet say 172.16.0.0 / 16 and wish to to use it for a Global Network. So I have split this initial network into four subnetworks,

Core Network 172.16.0.0,
one for EMEA 172.16.64.0,
one for AMER 172.16.128.0
one for APA 172.16.192.0.

Then I seperated the AMER in four regions by splitting the subnet again using two bits
North America 172.16.64.0
South America 172.16.80.0
Canada 172.16.96.0
Islands and Offshore 172.16.112.0.

But did I got wrong here, I then wanted two additional subnets for North American which would connect to the North America Core Router, here I used only one network but to subnet, should I have used two Network Bits.

Then I subnetted North America into two subnets i.e.
New York 172.16.64.0
Dallas 172.16.72.0

Question is... Have I gone wrong at this stage, should I have split say North America into four Subnets. I.E. what IP would I assign to you assign to say a NY router to connect to the North America Router which would connect to say the AMER core routers. As I have basically subnetting the network but not left myself subnets for the routers to work within.

Thanks

I have attached my full ip scheme.

chmorin

When dealing with VLSM there is a process that you need to go through. You can't just splice certain parts of it up.

You need to figure out how many subnets you need of each space, then go through and do the math. I usually organize them by the biggest needed subnet first, then work my way down.

Random example:

After figuring out the needs of your network, write out the amount of addresses you need. Like this:
172.16.0.0/16
Subnet A: 60
Subnet B: 60
Subnet C: 20
Subnet 20
Subnet E: 20
Subnet F: 10

Then one by one go through and find out what mask is best for those subnets, and find out where it leaves you, and continue after that point.

In this case:

A:172.16.0.0/26 255.255.255.192 172.16.0.1 - 172.16.0.62 172.16.0.63
B: 172.16.0.64/26 255.255.255.192 172.16.0.65 - 172.16.0.126 172.16.0.127
C: 172.16.0.128/27 255.255.255.224 172.16.0.129 - 172.16.0.158 172.16.0.159
172.16.0.160/27 255.255.255.224 172.16.0.161 - 172.16.0.190 172.16.0.191
E: 172.16.0.192/27 255.255.255.224 172.16.0.193 - 172.16.0.222 172.16.0.223
F: 172.16.0.224/28 255.255.255.240 172.16.0.225 - 172.16.0.238 172.16.0.239

Scale this up to your larger scale, and you should get it.

Hope that helped.

anfearr

Hi Chmorin.

Thanks for the reply, I see my error alright, I became bogged down in the idea of four subnets over the defined proceess, one additional question, is there any need for the networks be to contigious, can I single use an subnet to create 128 /30 subnets and use them to address my wan links throughout the entire network or would this be considered bad design.

e.g. all wan links within the 172.16.254.0 / 24 subnet.

Thanks

PhildoBaggins

VLSM requires large to small or top to bottom ip planning.

Always use the largest subnets first, following by the 2nd largest, 3rd, 4th etc...