Do I need to use shared-networks or not with Kea DHCP?

What are shared networks? When should you use them?

Shared networks are a way to tell the DHCP server that all of the subnets specified inside a single shared-network can be considered equal; IP routing of clients should not be broken if you give any client requesting an address, any IP address from any of the included subnets.

That's the very simple case - and it's pretty clear that if you ran out of addresses for a specific site and needed to add a new subnet for it, running over the same wire and sharing the same local relay, that this is a primary use case for shared-networks; it doesn't matter which subnet provides the address to a client, any/all should work just fine.

But there is more to this. Clients in a shared network are those who all reach the DHCP servers via the same route - be it a local interface, or, more likely, the same relay. Consider the case of cable modem provisioning where different types of 'client' will request addresses - all via the same relays, but with the need to distinguish between them and allocate them to different subnets. Typically it will be possible to separate clients by means of Classification and then use this to restrict clients within a shared-network to the subnets that they should be using. This is the second, and slightly more complex use case.

The basic principles are therefore:

• all the clients are reaching the DHCP server via the same local relays (or are local to the DHCP server and accessing it via the same local interface)
• unless classification is added, it's OK for a client to get an address from any subnet within the shared-network.

In all of the above, there is no mention of Host Reservations. They can and will of course change how addresses are allocated to clients, but having understood the basics of Shared networks and Classification, now reading the documentation on what's different when there are also Host Reservations to consider shouldn't be too difficult.