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# Subnet a subnet

Posted on 2014-10-19
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We have a MPLS network our ISP provides us with. It is a /20 network.
Now we want to have several vlans but we don't want to tell our ISP every time we need a vlan so we want to create all our vlans on our Layer 3 switch.

Our ISP tells me they will on their router point the entire /20 network to our Layer 3 switch. And so we can subnet the /20 network the into the subnets we like.

So I guess as long as I subnet using  /21 /22 /23 /24 this is possible?
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Question by:Bes4dmin
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Expert Comment

ID: 40390889
Yes that is correct.  If you have one connection on the MPLS network that the provider is routing a /20 to .. you can chop that up however you see fit at that location.

Now if you have 8 MPLS locations and the /20 is already chopped up between all locations that is a different story.  But it sounds like they are routing a /20 down a single network connection to a single location.
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Author Comment

ID: 40390906
we have six MPLS locations and each one of them have a /20 address. E.g one site has  172.16.80.0/20 and the other has 172.16.96.0/20
So every site can take after us then and start subneting. :-)
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Accepted Solution

Akinsd earned 500 total points
ID: 40391383
Let's categorize the ranges given to you are supernets, and we need subnets within each supernet

Supernet 1 (Site1) = 172.16.80.0 /20 means you can have addresses between 172.16.80.1 - 172.16.95.254, but Actual range = (172.16.80.0 - 172.16.95.255)
Supernet 2 (Site2) = 172.16.96.0 /20 means you can have addresses between 172.16.96.1 - 172.16.111.254 but Actual range = (172.16.96.0 - 172.16.111.255)
Site 3  range = 172.16.112.0 - 172.16.127.255
Site 4 range = 172.16.128.0 - 172.16.143.255
Site 5 range = 172.16.144.0 - 172.16.159.255
Site 6 range = 172.16.160.0 - 172.16.175.255

Both of these gives you 4,094 hosts each
The standard recommendation is a maximum 510 hosts per subnet for optimum performance anyways (Speed, congestion and collision are important factors).

What you'll nee to do is Subnetting. To do these, you will use notations greater than the one your ISP gave you, eg /21, /22, /23, /24, /25, /26, /27, /28, /29, /30, but you must still confine the address within the range you were given eg, Supernet 1 = minimum subnetted address - 172.16.80.1; Max - 172.16.95.254

Let's assume you plan to have about 500 hosts in your network, you will use /23 (255.255.254.0) which gives you 510 hosts
So your first assignable address in subnet 1 = 172.16.80.1 /23. Last assignable  Address = 172.16.81.254 /23
subnet 2 = 172.16.82.1 /23 - 172.16.83.254 /23
This continues up to subnet 8 = 172.16.94.1 /23 - 172.16.95.254 /23

If you think 1 subnet would not need more than 254 addresses, then you can use a /24 notation. This will give you up to 16 subnets within your supernet.
first assignable address in subnet 1 = 172.16.80.1 /24. Last assignable  Address = 172.16.80.254 /24
up to subnet 16 = 172.16.95.1 /24. Last assignable  Address = 172.16.95.254 /24

/23 (255.255.254.0)
Site 1                                          Site 2
172.16.80.1 - 172.16.81.255      172.16.96.1 - 172.16.97.255
172.16.82.1 - 172.16.83.255      172.16.98.1 - 172.16.99.255
172.16.84.1 - 172.16.85.255      172.16.100.1 - 172.16.101.255
172.16.86.1 - 172.16.87.255      172.16.102.1 - 172.16.103.255
172.16.88.1 - 172.16.89.255      172.16.104.1 - 172.16.105.255
172.16.90.1 - 172.16.91.255      172.16.106.1 - 172.16.107.255
172.16.92.1 - 172.16.93.255      172.16.108.1 - 172.16.109.255
172.16.94.1 - 172.16.95.255      172.16.110.1 - 172.16.111.255

/24 (255.255.255.0)
Site 1
172.16.80.1 - 172.16.80.255
172.16.81.1 - 172.16.81.255
172.16.82.1 - 172.16.82.255
172.16.83.1 - 172.16.83.255
172.16.84.1 - 172.16.84.255
172.16.85.1 - 172.16.85.255
172.16.86.1 - 172.16.86.255
172.16.87.1 - 172.16.87.255
172.16.88.1 - 172.16.88.255
172.16.89.1 - 172.16.89.255
172.16.90.1 - 172.16.90.255
172.16.91.1 - 172.16.91.255
172.16.92.1 - 172.16.92.255
172.16.93.1 - 172.16.93.255
172.16.94.1 - 172.16.94.255
172.16.95.1 - 172.16.95.255

Bottom line, 2 key factors
How many subnets do you desire in each supernet
How many host do you desire per subnet

In my examples above,
/23 will give you 8 subnets and 510 hosts per subnet.
/24 will give you 16 subnets and 254 hosts per subnet

I think you get the logic
subnets double while hosts breaks in halves as you go up the notation ladder

I hope this helps
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Author Closing Comment

ID: 40391596
Thank you! This is topclass educating answer and I will have it in my Q&A
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Expert Comment

ID: 40404367
Thanks for the kind words

All the best
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