Multiple subnets, wired/wireless

Niall O'Reilly Niall.oReilly at
Mon Sep 28 21:51:45 UTC 2009

Ashley M. Kirchner wrote:
>    I'm currently running FC10 with isc-dhcpd-4.0.0 serving one subnet.  
> I have to add a wireless router to the network now and want to have the 
> DHCP server on this machine to do the IP assignments (I'll be turning it 
> off in the router).  However, I also need to have a completely separate 
> subnet for those machines connecting to the wireless.
>    Right now our current subnet is an internal network 192.168.100.x  
> The wireless will be serving 192.168.200.x  The two DO NOT need to talk 
> to one another, they're going to be completely separate.
>    Can DHCPd actually do this, serve two separate subnets and know what 
> IP to hand out where and when.  

	Sure, but you have to configure everything consistently:
	network topology, network equipment, and DHCP server.

	Where I work, we have many access networks whose gateway routers
	act as DHCP relays, configured to catch the broadcast DHCP
	requests, forward them to the (ISC v3.1.0) DHCP servers, and
	steer the responses back.  It all "just works".

	At home, I have several NICs on an old Fedora 8 box running
	ISC dhcpd 4.1.0.  I use this box as my router, the star-point
	of my network.  Connected to this are a CAT-5 hub, a wireless
	bridge, and a wireless router, each to a different NIC.  This
	particular wireless router doesn't relay, but does NAT and
	provides its own DHCP service.  I leave that alone, as it's not
	configurable for interoperation with my DHCP server.  The other
	networks (copper, wireless bridge) are directly connected to
	different NICs on the Fedora box, and so don't need relaying.
	This is quite different from what we've set up at work, but also
	"just works".

	You can probably work out what you need for your setup by
	reading the excellent documentation provided with the ISC
	release,  and building a configuration to match the
	capabilities offered, and constraints imposed, by the equipment
	and network you have.  If you're down that path already, and
	stuck, I or others on the list will be glad to help, as long as
	we feel that you're being reasonable in what you ask.  Our time
	is precious too! 8-)

	I hope this helps a little.

	Niall O'Reilly
	University College Dublin IT Services

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