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How does IPv4 Subnetting Work?

We currently have a: Subnet.

We are looking to make this subnet larger as we need about 500 IP addresses in the above range.

How should we do this?

marked as duplicate by HostBits, EEAA, Khaled, Alex, Dave M Jan 28 '13 at 16:01

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • There are only 255 addresses in the above subnet. To gain more addresses, you will either need to make a larger subnet, say a /23 or add an additional routed subnet. – EEAA Jan 28 '13 at 14:46
  • You should consider instead creating more than one routed network rather than one huge bridged network. – David Schwartz Jan 29 '13 at 3:40

As others have answered the question, I won't bother to repeat what has been said. However, be sure you understand the ramifications of expanding a subnet to support 500 hosts.

There will be a lot more broadcast 'noise' on this subnet. Each ARP request will be seen by all 500 hosts, as will every DHCP request and various other broadcasts. As you have more devices, you will also have a greater volume of broadcast traffic.

On top of that you have unicast flooding performed by switches.


You would have to redo the network settings on all machines to use a /23 instead of a /24. Or you solve it in layer 3 and use two /24's and a router.

  • If we change all of our SN's to be /23 - would our range: still work and be part of the new range – Adam Chetnik Jan 28 '13 at 14:47
  • Yes, your range would then be – Dennis Kaarsemaker Jan 28 '13 at 14:58

Set your new IP network based on the max range of 500 IP addresses. If you use any Supernet calculator, you may get the following result as an example -

CIDR Address range - -

  • this will allow you to keep your original net block, except that you have to change the netmask to /23, and provide the new hosts IP address within the

Net CIDR notation (netmask) -

So you are basically extending the netmask to /23. By the way, this new subnet will give you a maximum of 510 addresses.

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