Engineering a new LAN IP Structure

Posted on 2011-04-27
Last Modified: 2012-05-11
Hi All,

I am looking at redesigning our private LAN ip structure. We have 26 sites and the subnets are arranged as We have from 192.168.0.X - 192.168.26.X subnets.

We have approx. 100 - 150 devices on each subnet. I was thinking of introducing a CLASS A network.

So The reason? I can use the second octet as a SITE ID, Third Octet as a CATEGORY ID (eg, servers, wireless, laptops, printers etc etc) and the fourth octet is range 1 - 254 for the devices themselves.

Now I like this as a clear identifier of subnets, eg, a wireless device isnt working on, I know this is site Silicon Valley(151), Wireless Device (60).

However, I am concerned that CLASS A is too large for only 150 devices or so in each subnet, is it overkill? What are the downsides of this in a technical level? Should I pursue a CLASS B instead? I am assuming that over the next 3 years we will grow by double, in which CLASS C will not be a viable subnet range.
Question by:Network_Padawan
    LVL 7

    Expert Comment

    We split down by category, but not at the level you are talking.. At the subnet mask you are talking the subnets would be huge...

    How about this?

    10.Global Region.SiteVlans.Device?

    So for example;

    Split world or country into 8 chunks (a /11 mask) - regions a-h or whatever you want to call them

    So sites in Region A would exist between and, B would be in to

    each site would become a /21 mask which would give you 8 .24 vlans on the site

    So site A in region A would to

    Within Site A you would have your 8 /224 vlans for your device types


    LVL 5

    Expert Comment

    I don't see any issue with this idea at all, they are after all just numbers available to use for whatever you need.

    Author Comment

    Well someone said that I shouldn't do it with class A because if I need to VLAN a segment, I need to use an entire different IP scheme, either 172.16.x.x or 192.168.x.x.

    Also, he was concerned that Class A would permit more chatter on the network, though what difference does CLASS A and B have to do with chatter if there are only 150 or so devices on the subnet anyways.

    I mean, you need to reach approx. 500 devices before you think of segmenting right?
    LVL 7

    Accepted Solution

    You can use a classless 10 network. You define it by the method I described (which I use globally!). If you define each site using 10..x.x.x/8 then you essentially putting each site on the same subnet. You don't want that. You can use a 10 network but using smaller subnets (classless).

    Also, by using the method I have given, each vlan is a /24 subnet so you are defining your broadcast boundaries as 254 devices..... I would never recommend anyone have a subnet containin 500 devices... your broadcast chatter would get redonkulous.....

    LVL 17

    Assisted Solution

    If you assume that you are routing traffic between sites, you cannot have the networks in each site overlapping each other. That means that you, at least, need to divide up the 10/8 to 26 different subnets, to start with.

    If you use 10/13 networks, you can have 32 of those, so if you never expect more than 32 sites that is enough, otherwise use /14 - or larger mask.

    You foresee up to around 300 devices per site, which means the smallest subnet mask you want is /23, which give you about 500 addresses per subnet.

    So, effectively you need to choose a subnet mask between /14 and /23 - these is not really any way around that.

    Now, it will probably make sense to choose a subnet mask that is easy to work with, and that is /16 that goes right between octets.

    You now have networks like:


    Possibly written like


    X will identify the site (and each site will have a unique subnet)
    Y will identify the device type
    Z will identify each device

    Y and Z will all be on the same subnet within each site.

    You can have 65000 usable addresses in each site.
    You can have 254 usable addresses for each device type, in each site.
    Of course, you can have Y=1 and Y=2 for PCs, giving 500 addresses for PCs.
    You can have 256 different sites.

    So, concluding, your own idea was great, but you just needed a different subnet mask, the /16.

    Only if you are not routing, can you use the /8.

    Author Closing Comment

    Thanks. Obviously I need to use /16. Not sure why I thought /8 was the best solution, its obviously not.

    Thanks guys.

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