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New subnet

I just started a new job and my first order is to create a new subnet.   The company has been working on the network for some time  and have used up all but 4 IP addresses.  I wanted to make a /23  network and put certain devices on the network.  I'm still  feeling out all the equipment that was there when I started.  One  router, one switch, no Vlans (small business).  I've played around with  it a bit but I'm having a hard time having any device on the 0.0/23  subnet be able to communicate with the 0.1/24 network or access any  network resources. 

Basically only certain devices will be on the 0.0/23 network but need to be able to access resources on the 1.0/24 network.  Only certain devices on the 1.0/24 network need to be able to access the devices on the 0.1/23 network (so far I've changed the subnet mask to on certain user devices on the 1.0/24 network and am able to access devices on the 0.1/23 network, but not vice versa). 


Re: New subnet

Whats your question?

And depending on your question, a list of devices and topology would help.

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New subnet


Your phrasing of the question suggests that you believe that is a network separate and different from Actually is part of network If you change the netmask on devices from /24 to /23 then they all should communicate without difficulty. Your post indicates that there are some problems in that communication. Can you verify that both devices that are having problems in communicating have /23 netmasks?

If the devices do have /23 netmasks and still do not communicate then you need to supply more information about how things are set up in this network.




Re: New subnet

Hi Shelby,

If you have a subnet of /23 then it contains and subnet too. You see, when you take that one bit from 24 mask thus making it 23 the subnet will double the size. means there are 512 addresses(510 usable).

So there has to be range from -

Therefore these two networks are overlapping - or to put in in another way - the /24 network is just part of the /23. Change all masks to /23 and make default gateway the same and that should resolve the problem.

Best regards,


Re: New subnet

"one router and one switch" and you've used 250+ addresses? How many actual devices are connected? Are you sure you don't have a dhcp lease problem?

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Re: New subnet

If you want them on separate subnets just make them both /24's , and . You would create subinterfaces on the router to trunk to the switch  on which you would create new vlans and setup trunking back to the router.  The switch has to be manageable and be able to create vlans and trunking. If its a unmanaged switch you are out of luck and it would have to be replaced  with something more advanced.  This gives you an idea.

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Re: New subnet

Thank you all for the ideas/suggestions.  I'm headed in the right direction and believe just needs some tweaking (and learning more about the current set up).  I'll definitely add in some VLANs as devices on the /23 subnet should only be accessible to certain people.  Thanks again. 

Community Member

Re: New subnet

On the router you have a statement that says ip address on the internal interface? Make sure you changed it from the /24 subnet mask you had previously. Replace the ip address above with your ip address that your clients use for default gateway.

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