Server platforms Toolkit

Static IP conundrum

Ray Geroski Tech Republic

Published: 01 Oct 2002 11:54 BST

Admins need to assign and track the static IP addresses required by many devices, such as servers and routers, so that other hosts do not duplicate these addresses. A seemingly ideal solution would be to use a table or database from which hosts can be assigned IP addresses. Further, if DHCP could then use this static IP address table or database, it could resolve not to hand out those addresses to other clients. But is that possible?

A reader recently posed this question in our Technical Q&A, offering details about why he needs such functionality. Although responses to his question didn't offer a tidy solution for employing DHCP in this manner, they nevertheless described a way to accomplish the task without using a table -- a DHCP feature that serves a similar purpose.

Populating host addresses from a table
Reader frankl_27 presented the challenge of how to use DHCP to assign host addresses stored in a table. He's running WinNT 4.0 (SP 6a) and wanted to use static addresses for specific systems that require monitoring or remote control access.

"I've recently been presented with the task of completely redesigning our network of 13 locations scattered across two states. All the machines are currently assigned static IP addresses (mainly due to remote monitoring and support needs)."

Frankl_27 wanted to know if it would be possible to use DHCP to assign the addresses based on the available IPs listed in the table.

"For example, I want to assign the PC Branch1President to 10.0.0.100, the PC SysAdmin to 10.0.0.68, the machine RouterA to 10.0.0.1, and all the other machines in the network in a similar fashion."

In addition, he wanted to overcome specific security challenges with the solution. If a machine attempted to make a DHCP request that's not specified in the table, the machine would be denied an address. This would prevent an unauthorised user from plugging a laptop into the network and obtaining an address that allowed a connection. Frankl_27 also wanted to be able to log connections with the MAC address, username, and date/time of login/logout. A key reason for this is keeping better tabs on the scattered locations he must maintain.

"I need defined IP addresses because I actively administrate all the workstations, servers, and routers from a single location. If the addresses changed all the time because of DHCP, my administrative abilities would be severely hindered."

The question frankl_27 posed makes sense from this perspective. Automatically assigning static IPs to machines from an existing table could save a net admin a lot of time and hassle in situations where the addresses needed to remain the same. He had a specific plan in mind -- he just wasn't sure if the existing protocol could accomplish what he needed.