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How will the dhcp server know which ip to assign to the clients

Posted on 2003-12-10
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hello

i am familiar with the DHCP service on windows 2000 server.

i have a question regarding the scenario represented at the link below

http://web.1asphost.com/fmh002/scenario.JPG

as it can be seen there is no dedicated router in the scenario. however the dhcp server has 2 nics installed, one connecting the 1st floor and the second connecting the second floor.

dhcp server has a superscope including the scopes : 192.168.1.0  and 192.168.2.0

how would the dhcp server know which is the correct ip to give to the computers on the different floors?

i am thinking of one scope for NIC 1 and one scope for NIC 2. but is this possible?

thank you in advance
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Question by:fmh002
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by:JFrederick29
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The DHCP server will assign addresses in the 192.168.1.0 subnet (I assume your mask is 255.255.255.0) out NIC1 and 192.168.2.0 out NIC2 because DHCP without the help of a relay agent will only respond to requests on the same subnet.  DHCP uses broadcasts and since you have two seperate subnets with a router in between (your server), broadcasts are not forwarded.
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by:fmh002
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so with that scenario the dhcp server is acting like a router because of the two NICs right?

so on the dhcp service is there an option of confugring a particular NIC to respond to a particular segment or is it just done by itself ?

and lets assume that the 1st floor clients are getting appropraite IP addresses and the 2nd floor ones too. now what should be done so that clients from 192.168.1.0/24 can communicate with clients on 192.168.2.0/24 and vice versa? can dns be installed on the same dhcp machine for this case?
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by:JFrederick29
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Yes, a computer acts as a router when having two network cards on seperate subnets.  DHCP does this automatically just by the nature of TCP/IP.  For example, a client computer on the first floor starts up for the first time.  It sends out a broadcast on the network requesting an IP address.  Only computers on the 192.168.1.0 subnet will process the broadcast.  NIC1 in the server is on the 192.168.1.0 subnet so it processes the broadcast and responds to the client with the appropriate IP address.

Routing is handled automatically in your scenario.  The server, in its routing table (you can see it by typing "route print" at the command prompt", will have a route to 192.168.1.0 and a route to 192.168.2.0 out the appropriate interface (NIC).  DNS only maps host names to IP addresses, it does not handle routing.
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by:chicagoan
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>so with that scenario the dhcp server is acting like a router because of the two NICs right?
The DHCP server is a program running on a computer. The DHCP server only responds to DHCP requests.
That same computer could run Routing and Remote Access Service, and function as a router.

>so on the dhcp service is there an option of confugring a particular NIC to respond to a particular segment or is it just >done by itself ?
If a NIC is in a broadcast domain that a DHCP backet is generated in, it will receive the packet.
If the DHCP service is configured to respond to requests on the subnet that this NIC is addressing, you'll get an address.

>now what should be done so that clients from 192.168.1.0/24 can communicate with clients on 192.168.2.0/24 and vice >versa?
The "gateway" in each subnet must be a router which has a route to the other subnet.
Traffic sent to ip addresses on the same subnet make use of ARP. Traffic sent to addresses on another subnet are sent to the gateway, which has a routing table that it uses to determin where to forward the traffic. If the dhcp server is multihomed, you can run a routing protocol on it to move traffic from one subnet to another.

>can dns be installed on the same dhcp machine for this case?
Windows 2000 does support running a DNS server. This resolves names for the clients and supplies them with the IP address of the target, it is then up to the gateway router to move the traffic onto the appropriate interface.
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by:fmh002
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ok i am clear with the questions i previosuly asked.

what if the scenario is as follows:

http://web.1asphost.com/fmh002/scenario2.JPG

assume the DHCP server has 2 ips bound to a single NIC. 192.168.1.0 and 192.168.2.0.

would the same take place as you mentioned above?



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by:JFrederick29
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Yes, same would apply exactly.
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by:lrmoore
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>assume the DHCP server has 2 ips bound to a single NIC.
>would the same take place as you mentioned above?

No. If all the broadcast dhcp requests hit the same NIC, then the most likely scenario is that only one scope will be used until it is used, then the next scope.

Windows 2000 will not route between subnets on a single NIC

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JFrederick29 earned 100 total points
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My bad, fingers were faster than the brain.  lrmoore is correct, it would not work as with the two NIC scenario.
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by:fmh002
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so there would be no logical separation of the network in the physically separated network (per floor) as desired in the scenario.

this is somehow troublesome. imagine if there are 10 subnets involved in a 10 story building, would installing 10 NICs on the Dhcp server be the solution? im sure there must be a way of tackling this. is my asumption wrong?
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by:lrmoore
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It's easy to do if you have the right equipment. Since you have different subnets, you need to route between them. Use a real router, not a server for routing, and you only need one DHCP server. Use VLAN's on the switches, and a high-speed L3 switch in the middle. The router can "proxy" DHCP requests per VLAN/interface to the one server. Instead of a broadcast packet, the DHCP server receives a proxy request "someone on subnet X needs an ip address" server then looks at scope for subnet X and responds with IP address. We do this every day with Cisco gear.
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by:fmh002
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>Instead of a broadcast packet, the DHCP server receives a proxy request "someone on subnet X needs an ip address" server then looks at scope for subnet X and responds with IP address

so then there is a way to configure a scope for a particular subnet then. how is this done? does it involve the MAC of the switches?
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by:JFrederick29
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No, say the DHCP server receives a proxy request with a source address of 192.168.1.10 (the computer requesting an IP address).  The DHCP server will assign an IP address from the scope for subnet (192.168.1.0).
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by:lrmoore
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The MAC of the switches has nothing to do with it.
Simply create a different scope on the server for each subnet. Each subnet scope will have a different gateway (the router/L3 VLAN interface), but you can have global attributes of domain name, nameserver, netbios node-type, etc, that apply to all subnets.
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