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Vlan Numbering Plan
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This article specifies a calculation method for assignment of VLAN numbers.

The objective is to correlate vlan numbers with any given subnet. Typically, the vlan number matches the third octet of the subnet used inside the vlan. However, this only works out when using full class C subnet allocations. If subnetting is used to further shrink the network allocation size (ex. using a /25,/26,/27,/28 bit mask) – correlating an associated vlan number is a little harder. In order to resolve this, a calculation can be used to assign all vlan numbers.

1st Octet 2nd Octet 3rd Octet 4th Octet
10 1 10 0

Using the example above, to assign the associated vlan number, use the following formula:

(256*(Subnet – 1) + thirdOctet)

Where Subnet = the iteration being used

Explanation:

If a class C mask (/24) is used then Subnet is 1

If the mask is greater than 24, then start by calculating how many subnets can be contained inside the class C. Once the network is subnettted, use the subnet number of the network being used.

Exceptions:

If the thirdOctet is zero, then use 255 as the 3rd Octet number

The numbering plan assumes the second octet will always be the same. Since this particular scheme only calculates using the third octet, it would produce duplicate vlan numbers, For Example: 10.1.10.0 & 10.2.10.0 This wouldn’t be a problem if we were using the two subnets on two totally different local area networks (ex: different offices, etc.) because you can reuse vlan numbers that are on separate switch domains.

Example:

10.1.10.0 will be further sub-netted using a /28 mask. For this example, the table shows the number of subnets available. With using the /28 or 255.255.255.240 mask, there are 16 available subnets available.

Iteration Network Iteration Network Iteration Network
1 10.1.10.0 8 10.1.10.112 15 10.1.10.224
2 10.1.10.16 9 10.1.10.128 16 10.1.10.240
3 10.1.10.32 10 10.1.10.144  
4 10.1.10.48 11 10.1.10.160  
5 10.1.10.64 12 10.1.10.176  
6 10.1.10.80 13 10.1.10.192  
7 10.1.10.96 14 10.1.10.208  

If we were using the 10.1.10.0 network then Subnet = 1
If we were using the 10.1.10.32 network then Subnet = 3
If we were using the 10.1.10.240 network then Subnet = 16

Final VLAN numbers for the three examples above:

10.1.10.0 = Vlan 10
(256 * (1-1) + 10) = 10

10.1.10.32 = Vlan 522
(256 * (3-1) + 10) = 522

10.1.10.240 = Vlan 3850
(256 * (16-1) + 10) = 3850

Author Info:

 
 
Scott's profession is a Senior Network Engineer at a Healthcare transaction company in Franklin, TN. When he is not trying to secure a network or come up with a design for a new project, he enjoys spending time with his family. You can find out more at: http://www.scottp.net

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One Response to “Vlan Numbering Plan”

  1. Mark Says:
    May 13th, 2011 at 7:28 am

    Interesting idea.

    I do have to point out a slight error in your math though. You need another set of parenthsis.

    Corrected Example 1 should be ((256 * (1-1)) + 10 = 10

    Your example would be (256 * (1-1) + 10) = 2560

    I thought I was going crazy trying to figure it out.

    Anyway, I have been trying to come up with something like this for a while . If you only have one class B in your switch domain it will work nice. I really want to use this method. So I guess I’ll have to stick to one class B per. Should be pretty easy though.

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