How to keep your IP address static with DHCP


One of the features we support with the NetEqualizer product is a Quota tool, which keeps a running count of total bytes used per IP on a network. A typical IT administrator wants to keep track of data on a per user basis over time, hence some form of Quota tool is essential.  However, a potential drawback of our methodology is that we track usage by IP.   Most networks use a technology called DHCP that dynamically hands out a new IP address each time you power up and power down your computer or wireless device. Most network administrators can track a specific user to an IP in the moment, but they have no idea who had the IP address last week or last month.  Note: there are authentication tools such as Radius or Nomadix that can be used to track users by name but, this adds a complex layer of additional overhead to a simple network.

Yesterday, when working with a customer, the subject came up about our Quota tool, and its drawback of not being able to track a user by IP over time, and the customer turned that into a teaching moment for me.

You see, a DHCP server will always try and give the same IP address back to the same device if the previous IP address is available.   So the key is keeping that IP address available; and there is a simple trick to make sure that this happens.

When you set up a DHCP server it will ask you the range of IP addresses you want to use. All one needs to do is ensure that the defined range is much bigger than the number of devices that will be on your network, and then you can be almost certain that a device will always get the same IP.  This is because the DHCP server only re-uses previously assigned IP addresses when all IP addresses have been assigned, and this would only happen if you defined your IP address range to a relative small number relative to the number of potential devices on your network.   There is no real extra cost for defining your DHCP address range as a Class B instead of the typical default Class C, which then expands your range from 255 to 64,000.  So make sure your ranges are large enough and feel free to track your users by IP without worry.

If you would like to learn more about our Quota tool, also known as “User Quota”, you can read more about it in our User Guide.

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