Gordon Messmer <[EMAIL PROTECTED]> wrote:
>> Maybe I should say "sort of" solved in that it works but I don't understand
>> why.
>Because DHCP requests and ack's are broadcast. The IP's don't make any
>difference.
>> The problem wasn't dhcpd or the dhcpd.conf file. We took another
>> look at the old server and the subnet mask the "real" device (eth0) was
>> correct for a class B network subnetted to class C (255.255.255.0).
>> However, the aliased IP device (eth0:0) had the subnet mask for the class B
>> (255.255.0.0). I don't understand why it works this way - can anyone shed
>> some light?
>
>If you were using a class B network, say 192.168.0.0, subnetted into
>192.168.76.0 and 192.168.78.0 using the above netmasks, your routing
>will still work correctly.
It seems to, but which netmask should be used? I'm assuming 255.255.255.0
since it's been subnetted into class C's. Maybe they can run DHCP for
their entire network on 1 server with 1 IP address since there's no
physical separation anyhow. For those PC's that should always be on a
specific subnet, they could use a host/MAC mapping which is what I do on my
network. Takes a little work to collect the MAC addresses from arp, but
once it's working it doesn't take much upkeep.
>Traffic sent to 192.168.76.XX is broadcast to the network, because the
>netmask includes that entire network. Traffice sent to 192.168.78.XX
>will also be broadcast to the network, and recieved by the appropriate
>machines. The problem, however, would lie in attempting to send data to
>192.168.75.XX, or 192.168.15.0, or any other class C in 192.168... The
>255.255.0.0 netmask may tell the kernel to broadcast these packets to
>the local net, and not to a router.
That's kind of what I was wondering about. Fortunately, the aliased IP
address serves no other purpose than DHCP - and since DHCP is a broadcast
type service instead of source/destination (as you mentioned), I'm
wondering if the alias is needed at all.
>Unless your router does reverse arp
>proxy, (and some do) then your traffic would be lost.
Dunno. Not my network or router. I'm not even sure the people that
maintain it know either since most of the router config is farmed out. The
only thing I do know about their router is the routers for the virtual
subnets are also "virtual" since there's no physical separation. (Or as my
friend likes to call them "a router on a stick".)
>Now, since DHCP requests are broadcast, and do not have src/dst
>addresses to speak of, routes, ip's, and netmasks don't have much
>effect. Both of your subnets speak to the same physical device, so the
>DHCP server will assign addresses from the same pool, unless the
>ethernet address is listed in the dhcpd.conf, and given a specific IP
>there. There is no other way for the DHCP server to know which subnet a
>given computer is on.
That's *exactly* what I was wondering about yesterday. Since the network
segments are not physically separated how would DHCP know which subnet a
computer is supposed to be on - it wouldn't. What I'm thinking is DHCP
will assign addresses out of pool A until there's no more left and then
assign from pool B. Is this correct thinking?
>Most subnetting schemes would involve two hubs (or pools of hubs) and
>two NIC's in your linux box/router. In this case you'd have eth0 and
>eth1, not eth0 and eth0:0. Because there are distinct physical devices,
>dhcpd knows which subnet should recieve what addresses.
Right. That's how I would have done it were it my network, but it isn't.
To be honest, I'm not really sure why their network is setup the way it is
- it seems a little goofy to me but apparently it works for them. To each
his own.
>Sorry if my previous post was a little curt, sometimes I'm hasty with
>the "Alt+Enter". If there's anything you'd like me to clarify, I'd be
>happy to.
No offense taken. You've already helped me to clarify a few things I was
wondering about - some of them before I had even asked the question. <grin>
Thanks!
-Eric
Eric Sisler
Library Computer Technician
Westminster Public Library
Westminster, CO, USA
[EMAIL PROTECTED]
Linux - don't fear the Penguin.
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