Newbie question: Servers for Multiple subnets on same Ethernet

Niall O'Reilly Niall.oReilly at
Fri Mar 16 12:54:28 UTC 2007

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On 16 Mar 2007, at 12:25, Chiu, PCM (Peter) wrote:

> We currently have a (windows based) dhcp server serving mainly laptops
> on our LAN.
> The LAN is made up of multiple gigabit links to a local Nortel router.
> We have three subnets overlaid on the LAN.
> We are going to introduce a block of Linux servers, and use one of  
> them
> as a master
> to serve operating system to the other Linux servers.

	You might want to do this on a back-end, server-only LAN.
	If your servers are like the ones we use, they come with two
	network interfaces on the system board itself.

> These servers
> will be connected on the same
> LAN (through a separate managed switch) so that they can be reached
> directly by users on the same LAN.
> If we enable the dhcpd service on the master, to serve the Linux  
> clients

	I suppose you mean, end-user machines, and not the other servers.

> a. using addresses on a different subnet from that served by the  
> windows
> dhcp server, and
> b. restricting the service to those with specific MAC addresses,
> is there anything we need to watch out for?

	Thiongs that come to mind ....

	If you configure your new DHCP server as "authoritative" (which	
	you'll likely want to do), you'll need to configure every one
	of your subnets within a "shared-network" block in your config.
	Otherwise you will find your new server contradicting the
	existing one by sending DHCPNAK for any address within the
	subnet(s) it's not aware of.  You don't need to configure hosts
	or address pools within the subnets the other server is to manage,
	just to make the new server aware that the "extra" subnets are

	You may want to use a pair of DHCP servers, set up with failover.

	It may simplify things if you can plan to retire the existing
	DHCP server and just manage one DHCP service on your LAN.
	DHCPINFORM will be an issue for this.

	Best regards,

	Niall O'Reilly
	University College Dublin IT Services

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