Subnetting A Private Address

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Mar 27th, 2008
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This should be basic, I want to create an additional subnet, but would like to leave the existing subnet unaltered. The existing subnet is To make a new subnet, can I use the same bit mask with an address out of the existing range? For instance, would be a valid subnet? Also, one I have the address range for the new subnet, how do I apply this? Do I need to apply an address from the subnet to an interface of a routing device, be it a core router or switch? If a DHCP server were involved, I would assume I would have to configure the scope of the new subnet. Any help or references on the topic would be appreciated.

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jeff.velten Thu, 03/27/2008 - 06:11
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That range would be fine for a private network, and yes, you'll need to route between the two. Just out of curiosity, why is your subnet mask a /22, rather than the more common /24?

QuikeyMan_2 Thu, 03/27/2008 - 06:43
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I have no idea why the original range was set to 22 as opposed to 24 mask bits. So, given that range would be fine, how would I go about applying it? Our network consists of, correct me if any of this sounds off, a cat. 3750 that routes to another cat. 3750, a cat. 2948, and to a cat. 3560. I would be able to better confirm this, but I am still tracking down the login credentials of the 3750.

derrickc Thu, 03/27/2008 - 07:48
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Depending on the number of hosts you have, you can subnet however you need to...Since already exists, it will include -

So, yes, the is valid, but would give you 1022 hosts. Do you need that many?

You could also do the following: (510 hosts) includes -

and of couse (254 hosts)

It just depends on the size you need.

You will have to setup one of your L3 switches to route between the VLANs. And if you want the new VLAN/Subnet available across your network, you will have to create the VLAN on any switch that will have a port in that VLAN as well as setup trunking between the switches. Not knowing your configuration, I'm not sure if this is already in place or not.

As for DHCP, you will have to setup different scopes for each subnet. Depending on your setup, you many also need to configure an ip-helper address for your new VLAN if the DHCP server resides on a different network.


QuikeyMan_2 Thu, 03/27/2008 - 08:21
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Thanks for the reply Derrick, and you are right in questing that amount of hosts, I will probably trim it back to /24.

Our setup consists of a 3750 that all other switches, including the 3560 that the new vlan will be on, are attached to. At this point I will not need the new vlan available to any other switches. I will only need to access one host on the new vlan from the existing subnet.

My hangup now is locating the credentials for the 3750 to change the configuration....


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