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DHCP and unauthorized users

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by mark_scott44

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My office shares a network backbone with two other offices in our building. I have implemented DHCP while they have not. When they set up new computers they are accepting the network defaults which automatically attempts to connect to a DHCP server.In short, they are "stealing" addresses from my assigned range.

Is there any way to block certain MAC addresses once they are discovered??

Thanks in advance

Mark

3 replies

by timwalsh

In short, no. DHCP is not designed to handle multiple entities on a single network segment. Likewise, if one of the other businesses were to set up a DHCP server, there would be nothing to prevent your company's computers from accepting IP addresses from another DHCP server. Because of the nature of DHCP, a DHCP client will accept an IP address from the first DHCP server that answers.

Who manages the building backbone? You may want to talk to them about segregating your company into a seperate subnet. You would probably need to provide additional equipment such as a router and hubs to do so.

Your only other route, barring getting the IT folks from the other companies to not connect a computer to the backbone until TCP/IP has been configured, and barring getting the building managment to allow you to segregate your segment from the rest of the building, is to use fixed IP addresses.

by wayne r.

Answer 1 is correct in so much that you can't block the other IP Addresses, however, another workaround if you want to keep DHCP is to add a reservation to all the addresses in your scope and as and when you have a new machine put the MAC address into one of the reserved addresses.
I know this removes some of the flexibility of DHCP but at least you will still have a single point of administration.

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