Superscopes

Dracula28

I understand the concept of Superscopes, but I'm curious about how its determined which clients receive addresses from which scope, when there are more than one logical subnets on the same physical network.

Using superscopes: Dynamic Host Configuration Protocol (DHCP)

In example 2 there are three scopes providing addresses to three different logical subnets on a single phyiscal network segment. The router is configured with three different ip addresses (will it need three different interfaces?). So how does the DHCP server determine from which scope it should assign addresses to which client? How does it know which client belongs to which logical subnet? Are user classes being used, or is this information retrieved from the router/switches?

chmod

Due to the dhcp relay, it forwards the broadcasto to another subnet(where the dhcp server resides), you tell each interface or subinterface that receives the broadcast from the clients where to forward those request and it replaces the ip address with the gateway ip address ant the DHCP server uses that information to allocate an IP within that subnet.

Dracula28

But there is no relay agent in Example 2. All three logical subnets are on the same physical subnet. So how does the DHCP server know which DHCP client is on which subnet? The client gets connected to the subnet, sends out a broadcast, the DHCP server receives it, but how does it know from which scope it should assign an ip address to the client?

ptilsen

A new DHCP client will pull from whatever scope has available IPs, and the scope chosen cannot really be configured or predicted. I'm not sure how it "chooses" and can't find it documented, but I would assume it is random or whatever has the most available IP addresses. An old client will request its old IP and generally get it.

Since a Superscope is really only useful when one wants to extend the size of a physical segment without reconfiguring the logical subnet, it is assumed that there is no need to put certain clients on certain subnets. If there were, you would need to physically (which includes virtually) segment them, at which point you would have some control over what computer gets what IP.

Side note: You'll almost never see a superscope in a production environment that isn't a misconfiguration by an admin who didn't understand it's purpose. It is generally well worth the hassle to either segment the network or reconfigure the logical subnet if IP addresses are running low. Superscopes are the lazy way out of a situation that doesn't happen that often.