Dual Nics for accessing network, and dedicated NAS

Here's my situation, I'm trying to setup a server with two nics, one of which is connected to the network, and the other connected to a NAS via crossover. I've never done anything with subnetting, but I was thinking I could just set the main nic to with a subnet of and the other to with a subnet of and the nas nic set to

Am I thinking through this right, or is there a better way to do this?

Thanks for any input!

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  1. Best answer
    Hi Mick,

    You are correct in that with this type of configuration you will want each NIC on a separate subnet. Doing this will prevent you from having unexpected routing problems out of your server. However, your proposed configuration doesn’t actually achieve this. Technically both of those addresses are in the subnet you defined for your primary NIC which will almost defiantly cause problems.

    Without going too far into subnetting (there are entire books on it if you are interested) a netmask mask of defines a subnet with 254 hosts. Every IP from to would be on that subnet. Technically, even and are on the subnet, but they are reserved for the network ID and Broadcast respectively. Because of this, using on the other NIC could confuse the internal routing of the server. If a packet was generated for, a possible address for your NAS, the server would not know if it should send it out NIC1 or NIC2 since that address is on both subnets. This could result in unexpected behavior.

    To fix this, you can set up the primary NIC as you described ( but you should set your NAS NIC to something like You could even use the same subnet mask of since we changed the value of the third octet of the address. You would then give the NAS any other 182.168.20.X address you like.

    If you really want to get fancy with the subnetting, you would use a netmask of which allows only two hosts. Since you are using a crossover cable there can only be two hosts. This could look something like:

    Server NIC2:

    When using a mask like this you need to be sure that you use the correct two addresses. For example, and would not work with a subnet mask because the .251 address is reserved as the broadcast address for that subnet.

    I hope you find this helpful. Best of luck with your project.

  2. Great info! Thanks!
  3. So I applied this setup, and found that when I have the 'backup network' enabled I loose internet access on nat system, I assume because it's trying to use its default gateway, which isn't connected to the internet. It also prevented incoming email, I imagine for the same reasons.

    Any thoughts? Thanks again!
  4. Make sure you have the default gateway for the internet side set correctly. On the non-internet side, don't set a default gateway. Just leave it blank. This will force all unknown iPs to go out the internet link.
  5. I think that got it (and makes sense too!). Thanks!
  6. Best answer selected by 229Mick.
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