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Wireless Networks Thread, using 2 dhcp servers on different ranges & domains in Technical; Hi guys, can anyone help me out here trying to install 2x thin client system when I boot on PXE ...
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    using 2 dhcp servers on different ranges & domains

    Hi guys, can anyone help me out here trying to install 2x thin client system when I boot on PXE it picks up the DHCP from the admin network, the client will be on my curriculam network (statically assigned IP's) of which I don't want them to have access to an admin IP, have tried in the past to setup both networks on DHCP but then got stuck when trying to figure out how to tell the client machine which DHCP server they should be pointing to, i'm on a windows 2003 enviroment with xp clients does anyone know if it's possible to specify which DHCP server IP a clients should direct to?

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    FN-GM's Avatar
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    You should only have one DHCP server per network. Unless they are of different vlans or totally different networks.

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    vlans

    In mutiple vlan scenarious you can still get away with using just one DHCP by having multiple scopes defined on it to cater for all vlans. The clients in each vlan will getthe ip from the correct scope providing the vlans are setup/configured correctly.

    Ash.

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    I think the thing to recognize here is that putting machine's on different domains dosn't mean they are on seperate networks (at least at a TCP/IP level). So there is no reliable way (that I know of) to tell the machine's which DHCP server to use so they will probably be allocated IPs randomly.

    DHCP is a very low level protocol and kind of half the point is that you shouldn't have to specify the server.

    One option is the VLAN route which does create seperate networks, although this may give you problems elsewhere and your equipment (switches, etc) has to support it.

    Another would be to use one DHCP server and assign ips based on mac addresses, or something.

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    By setting the boot server flag in the DHCP settings, it's not going to affect any machines other than those set to boot from the network.

    I'd just alter the current server. If people decide to fiddle in the BIOS and boot up a thin client, then they'll just have the access they're normally allowed, anyway.

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    petectid's Avatar
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    Quote Originally Posted by FN-Greatermanchester View Post
    You should only have one DHCP server per network. Unless they are of different vlans or totally different networks.
    Not strictly true FN-Greatermanchester you could divide your scope and spread it across several servers if you wish.

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    Andi's Avatar
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    Quote Originally Posted by ashok View Post
    In mutiple vlan scenarious you can still get away with using just one DHCP by having multiple scopes defined on it to cater for all vlans. The clients in each vlan will getthe ip from the correct scope providing the vlans are setup/configured correctly.

    Ash.
    That's a hot topic in my mind at the moment. How does DHCP know what IP range to send to a client?

    EG. If I have one DHCP server with multiple scopes for all the various vlans (all on different subnets) how does the DHCP server know what IP range to give to a new machine being plugged into vlan2?

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    Geoff's Avatar
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    If I have one DHCP server with multiple scopes for all the various vlans (all on different subnets) how does the DHCP server know what IP range to give to a new machine being plugged into vlan2?
    You must use a DHCP Relay on your router to make this work. See RFC 1542 for more information.

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    Jona's Avatar
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    I assume based on the mac address, of the machine, some may support a default vlan, if the mac address dosn't match. I'm sure someone will correct me if I'm wrong.

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    ip helper command

    Quote Originally Posted by Andi View Post
    That's a hot topic in my mind at the moment. How does DHCP know what IP range to send to a client?

    EG. If I have one DHCP server with multiple scopes for all the various vlans (all on different subnets) how does the DHCP server know what IP range to give to a new machine being plugged into vlan2?
    Hi There,

    The way to do this would be to use the ip helper command (in cisco, other vendors probably have a diffrerent command for it) and this will tell the vlan i.e. for vlan 1 config you could have ip helper <ip address> of the DHCP server and same for vlan 3 etc, the ip helper ip address will be the same for all vlans.

    The way it know which scope to assign an ip from is by inspecting the gaddr so when the dhcp request reacched the vlan interface it will be a broadcast but when it is sent to the dhcp server the packets are converted to an unicast address and because the packet came from vlan 1 say (e.g. valn 1 range is 192.168.1.x) then the dhcp server will know which scope (i.e. 192.168.1.0) range to allocate the ip address).

    HTH,

    Ash.

  11. Thanks to spc-rocket from:

    Andi (6th February 2008)

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    Andi's Avatar
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    So essentially it'll be a configuration option somewhere on my procurve backbone? Ok thanks.

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