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Assign a DHCP address on a different subnet to the source address.


I have a private mobile APN from Orange UK. It has become a requirement for us to assign device IP addresses to the mobile clients from our central DHCP server. The problem is the DHCP requests will be sourced from an address that is not part of the required handset IP range.

For example, The DHCP request will be sent from a device in the Orange network with a source of 192.168.x.x but I need to assign the client address from a 10.x.x.x pool. Does anybody have an Idea if this is possible with DHCP? It was my understanding that the DHCP server will always look for a scope that matches the requesting address?

Many Thanks

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Re: Assign a DHCP address on a different subnet to the source ad


Let me see if I understand you correctly:

When a client is looking for an IP address it will send a broadcast DHCPDISCOVER to all 255s with its IP set to and it will be blocked by routers.if the DHCP server is on a different subnet that the client then for the DHCPDISCOVER  to get to the server you need a DHCP Relay which will receive the broadcast and will send it as unicast to the server.

Once the server gets this unicast DHCPDISCOVER it will look for the giaddr where the IP of the DHCP Relay interface which received the broadcast is located and will look for a scope in this subnet to offer.

So the DHCP Relay seems to be what you are looking for. On a Cisco router this is implemented with the interface command: ip helper-address x.x.x.x where x.x.x.x is either the IP address of the server or the broadcast address of the subnet wher the server resides.The interface it is configured on is the interface which receives the broadcast DHCPDISCOVER.



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Re: Assign a DHCP address on a different subnet to the source ad

Thanks for your response Alain,

That is how I understand the DHCP process to work.

The problem I have is that the client DHCP request will be intercepted by a router on the Orange network. This is configured to forward the request to our DHCP server but the GIADDR will be of the Orange router (a 192.x.x.x address).

This means that the DHCP server will receive a DHCPDISCOVER with a 192 address in the GIADDR field.

I need to somehow tell the DHCP server that any requests that are received from the Orange router on a 192 address are assigned an address from a specific 10.x.x.x range. Any Ideas?



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