Separates VLANs with the same IP-Range

glenn.satchell at glenn.satchell at
Mon Apr 29 06:14:02 UTC 2019

Remember the DHCP relay only handles relaying broadcast traffic from the 
client. This is the DHCPDISCOVER and initial response packets. Once the 
client gets an IP address it talks directly to the DHCP server for 
checking, renewing and acknowledging, the relay is no longer involved.


On 2019-04-28 02:02, Simon Hobson wrote:
> german181 at wrote:
>> So i remove the nat konstrukt and add a subnet for the second relay.
> It's not entirely clear what your network topology is here. Are you
> saying that there is NAT between the DHCP server and the clients ?
> There must be no NAT between clients and server - the server needs to
> be able to send unicast packets to the client and this cannot be done
> if there is NAT in the path.
> Also, you cannot have any overlap in IP addressing between networks -
> normal packet routing doesn't handle this, and the ISC DHCP server
> certainly doesn't.
> Remember two two fundamental rules of IP are "addresses must be
> globally unique"(1) and "any node should be able to address a packet
> to any other node"(2). NAT breaks both of these.
> 1 - In the context here, in the collection of networks your DHCP
> server is to handle, all addresses must be unique.
> 2 - Obviously these days subject to administrative restrictions (ie
> firewall filters). But again for your context, the DHCP server and any
> client it is to support must be able to address unicast packets to
> each other and have them delivered.
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