DHCP Reservations and Exclusions

by Chris Sanders [Published on 20 Sept. 2007 / Last Updated on 20 Sept. 2007]

When configuring a DHCP server I have often seen people interchange the words reservation and exclusion. These are two incredibly different concepts.

I have seen people interchange the words reservation and exclusion many times when talking about the configuration of a DHCP server. These two concepts sound similar at first but they are actually quite different and serve their own purpose.

An exclusion is an address or range of addresses taken from a DHCP scope that the DHCP server is not allowed to hand out. For example, if you have set a DHCP server to exclude the address range 192.168.0.1-192.168.0.10 then the only way a computer on your network would get an address of 192.168.0.4 would be if you assigned it statically on that machine. This is because DHCP knows NOT to give this range of IP addresses out.

A reservation is a specific IP addresses that is tied to a certain device through its MAC address. For example, if we have a workstation on the network that requires a certain IP address, but we don’t want to go through to trouble of assigning it statically, then we can create a reservation for it. So if the MAC address of the NIC on the computer is AA-BB-00-FF-CC-AA and we want it to maintain the IP address of 192.168.0.100 then we would create a DHCP reservation under that particular scope saying that the IP address 192.168.0.100 is reserved only for the MAC address AA-BB-00-FF-CC-AA.

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Chris Sanders is a network consultant for KeeFORCE, one of the most popular network consulting firms in western Kentucky. Chris is the author of the book Practical Packet Analysis as well as several technical articles. His personal website at www.chrissanders.org contains a great deal of information, articles, and guides related to network administration, network security, packet analysis, and general information technology.

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