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Wired Networks Thread, Changing subnet from /24 to /16 in Technical; Just need to check my logic on this one... I've got two networks at the moment... admin on a standard ...
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    gshaw's Avatar
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    Changing subnet from /24 to /16

    Just need to check my logic on this one...

    I've got two networks at the moment... admin on a standard Class C /24 which is fine for what it is at present, then a teaching network which bizarrely has a class A subnetted as a C (no idea why)

    Problem being we've hit 200 odd teaching workstations, 20 odd printers, wireless going in and laptops going onto the network so 254 hosts is going to run out of room very soon. In the long-term I want to merge both networks together and use a bunch of /24 VLANs routed via our 5406zl. That won't happen just yet due to some other issues so need to do enough to keep the teaching network going for a while longer.

    So to keep amount of reconfiguration as low as possible I'm thinking of changing the mask to 10.0.0.0/16 so I get 65534 host addresses. In theory as well I should be able to leave servers etc with IP addresses as they are (10.0.88.x) and just change the subnet over to /16. Then just make a new DHCP scope anywhere within that range (need to migrate the role to a 2008 R2 VM anyway).

    Seems like changing subnet doesn't affect DCs etc as much as shifting IP addresses so apart from the manual re-config of printers etc shouldn't be too painful. Input and experiences welcomed

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    You're confusing me a little with your Class A/C comments. That's old school terminology which may or not reflect what you are actually describing. So to be clear, you currently have a 192.168.x.x /24 and a 10.0.0.0 /24 network? Your problem is that you are running out of IP space on the 10.0.0.0 network due to some new developments. A /16 would work for what you are looking to do, but that is a large broadcast domain. It really depends on your network and the protocols you're running, but you typically want to keep the number of hosts per VLAN under 250 if you can.

    If wireless is "going in" I would put that on a separate VLAN from the start. This will allow customization for performance and security. Depending on your system, it may be a pain to modify the access points down the road. If you need to get your network up and running without configuring new VLANs the setup in your post will work locally. However, make sure any routing changes are carried over to the WAN.

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    Invest time and effort at this moment. Your network will get bigger and bigger. I would reconfigure this network. Create separate VLAN's for Wireless, Servers, Workstations, printers. Thus creating /24 or /23 networks. So it will be easier to do it now then in the future.

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    bio..

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    Thanks for the replies so far. Just to clarify it a bit...
    @SuperfluousAdjective yup it's a 10.0.0.0/24 which is pretty much on the limit for hosts at the moment. Granted the /16 isn't ideal but should do as a sticking plaster just to see us through for a couple of months.

    The multiple /24 VLANs with routing is my ultimate aim, along with removing the two networks but this depends on business processes changing for me to be able to use a single AD (it's complicated )

    Don't really want to mess with routing across VLANs this summer, first part of the wireless is just to extend the network to our institutionally-owned laptops so those can drop in with the standard desktops for now. Have already created an additional non-routed VLAN for guest access with isolation policies etc.

    Not too much of a problem to change VLAN settings as it's a managed Ruckus system
    Last edited by gshaw; 30th May 2012 at 12:16 PM.

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    Quote Originally Posted by gshaw View Post
    Thanks for the replies so far. Just to clarify it a bit...
    Well good luck to you. Don't forget to make sure nobody else on the WAN is using that IP space in any way. 10.0.0.0-10.0.255.255 is a large IP space to move yourself into.

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    Quote Originally Posted by gshaw View Post
    Thanks for the replies so far. Just to clarify it a bit...
    @SuperfluousAdjective yup it's a 10.0.0.0/24 which is pretty much on the limit for hosts at the moment. Granted the /16 isn't ideal but should do as a sticking plaster just to see us through for a couple of months.

    The multiple /24 VLANs with routing is my ultimate aim, along with removing the two networks but this depends on business processes changing for me to be able to use a single AD (it's complicated )

    Don't really want to mess with routing across VLANs this summer, first part of the wireless is just to extend the network to our institutionally-owned laptops so those can drop in with the standard desktops for now. Have already created an additional non-routed VLAN for guest access with isolation policies etc.

    Not too much of a problem to change VLAN settings as it's a managed Ruckus system
    So why not give it a 23 bit mask if it is just a sticking plaster until you get your vlan routing fixed.
    Expanding to 65k addresses is insane. It is going to create more headaches in the long term.
    10.0.0.1 - 10.0.1.254 would suit better

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    gshaw (30th May 2012)

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    Fair point, only reason for the /16 was that it wouldn't need any IP addresses changing but seems the negatives might outweigh that... back to the drawing board for a bit methinks...

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    Quote Originally Posted by gshaw View Post
    Fair point, only reason for the /16 was that it wouldn't need any IP addresses changing but seems the negatives might outweigh that... back to the drawing board for a bit methinks...
    You wouldn't need to change any IP addresses with a 23bit mask either

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    a teaching network which bizarrely has a class A subnetted as a C (no idea why)
    I'd warrant that it's the same as my place - it's a legacy from when your IT was part of the LEA. We were on a 10.0.0.0/20 range (although I've recently begun subnetting that down to /24) that was told to the school when it first joined the county-wide forest.

    As to the /16 idea - I realise that is 65k addresses but it's not going to be 65k hosts overnight, is it? I don't see how it makes any difference what size range you have if it's the same number of hosts on it (although there's no point in going so big if you know what size you need, I suppose)

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    Quote Originally Posted by bio View Post
    Invest time and effort at this moment. Your network will get bigger and bigger. I would reconfigure this network. Create separate VLAN's for Wireless, Servers, Workstations, printers. Thus creating /24 or /23 networks. So it will be easier to do it now then in the future.

    regards
    bio..
    This is what I would do.

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    Quote Originally Posted by CyberNerd View Post
    You wouldn't need to change any IP addresses with a 23bit mask either
    One snag is that the current subnet is 10.0.88.0/24 rather than what you might expect as 10.0.0.0/24, hence the /16 leaves 10.0.88.x in place.

    Yes the VLANs are the better solution and yes it's where it will go once we have a larger network redesign (already starting on this getting FIM etc in place) but if I do that now it's going to give a headache with the other network that's also running at the moment. Will need routing setting up on the core then making sure only the teaching VLANs can communicate with each other... a lot of work for a temporary fix. Too many changes in one go for this summer with everything else going on tbh.

    Using the /23 suggestion I could have 10.0.88.1-10.0.89.254? That'd do us for now
    Last edited by gshaw; 6th June 2012 at 09:49 AM.

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    Quote Originally Posted by gshaw View Post
    Using the /23 suggestion I could have 10.0.88.1-10.0.89.254? That'd do us for now
    Calculator says yes

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    Quote Originally Posted by sonofsanta View Post
    Calculator says yes
    Hehe the joy of subnet calculators, network tech's best friend

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    Hi guys,

    I have a question regarding changing subnets in DHCP. I know I will need to delete the current scope and add a new one with the new subnet.
    However, do I need to make any changes to the DNS server?
    I'm using Server 2008 R2 btw

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    Quote Originally Posted by CyberNerd View Post
    So why not give it a 23 bit mask if it is just a sticking plaster until you get your vlan routing fixed.
    Expanding to 65k addresses is insane. It is going to create more headaches in the long term.
    10.0.0.1 - 10.0.1.254 would suit better
    Would have to agree with this. Also, your DNS shouldn't need any alteration as long as your DNS Servers retain the same addresses.

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