Creating and configuring superscopes and multicast scopes
The majority of the scopes that you configure on a DHCP server are going to cover traditional IP address ranges. Occasionally, it will be necessary to configure a superscope to collect scopes together or to configure a multicast scope to address the needs of a specific application.In this section, you’ll learn about both scope types and the sorts of problems that they can help you solve.
DHCP superscopes are useful when you have more DHCP clients on a network segment than your DHCP scopes can handle individually. For example, let’s say you inherit a network segment assigned the 192.168.1.0/24 address range, with a DHCP scope range of 192.168.1.21 – 192.168.1.254, and the first 20 addresses reserved for static assignments. As the number of DHCP clients on this segment grows and approaches the upper limit of 234, you might see clients failing to obtain an address because no leases are available when needed. When the number of clients surpasses 234, it will become impossible for all clients to have an address at the same time. You need more addresses, for the network, but the scope as currently configured is exhausted.
DHCP superscopes provide a solution to this problem. A superscope allows you to create a second, spillover scope that will be used to provide addresses when the first scope no longer has any addresses available to lease out to clients. For example, in the previous scenario, you can create a second scope in the 192.168.2.0/24 range and roughly double the number of leasable addresses on the network segment.
Superscopes are necessary because if DHCP server has only one network interface, it can only provide addresses from DHCP scope to clients on a specific network segment. If you just created an additional DHCP scope, only the scope corresponding to the DHCP server’s primary address will be used. The superscope unifies two or more scopes into one scope so that more than one scope can provide address leases to clients if necessary. You can also use superscopes to add non-contiguous ranges together, so if you are using 192.168.2.0/24 on another segment already, you could add 192.168.3.0/24 to 192.168.1.0/24 when creating a superscope.
There’s one slight complication about using two scopes that we’re glossing over here. If you’ve used up all the addresses in a first logical subnet (for example, 192.168.1.0/24), then any new scope you add to the same network segment will, of course, be in its own logical subnet, distinct from the first. The second logical subnet will therefore only be able to communicate with the first subnet through a router; also, the second subnet will require its own corresponding default gateway address, such as 192.168.2.1. (It’s recommended that you use a second network interface on the router to enable this routing between logical segments,but some routers, including the Routing and Remote Access service in Windows Server 2012 and Windows Server 2012 R2, let you assign the two addresses to the same network interface.)
NOTE MIGRATING CLIENTS WITH A SUPERSCOPE
Another reason to use a superscope might be if you need to migrate clients gradually into a new IP numbering scheme. To do this, you can create an exclusion range covering the entire first scope, so that when clients renew leases, they will automatically receive leases in the new scope. When no more addresses in the old subnet are leased out, you can retire the old subnet.
To create a superscope, first create all the scopes that you want to include in the superscope.Then, in the DHCP console navigation pane, right-click the IPv4 node and select New Superscope from the shortcut menu. You then specify which scopes you want to add to the superscope, as shown in Figure 4-1.
FIGURE 4-1 Selecting scopes
Important: If an exam item mentions running out of addresses in a scope, or mentions needing to merge scopes together, remember superscopes.
Certain applications support multicast, which uses a specific segment of the IPv4 address space to allow IP hosts to communicate with groups of other hosts efficiently. With multicast,applications send data to one of these special IPv4 multicast addresses and all of the clients (and only the clients) that have subscribed to that address receive the data. An example of an application that can use multicast is Windows Deployment Services (WDS).
The protocol by which clients request and automatically obtain a multicast address is not DHCP but Multicast Address Dynamic Client Allocation Protocol (MADCAP). Despite this technical difference, MADCAP is handled by the DHCP Server service in Windows Server and it’s configured in the DHCP console. As with DHCP for unicast, MADCAP isn’t required for multicast. You can also configure multicast addresses manually on individual clients.
Configuring MADCAP on a DHCP server amounts to creating a multicast scope, which is easy. In the DHCP console navigation pane, right-click the IPv4 node, and then select New Multicast Scope from the shortcut menu. You will be prompted to name the scope and give it an address range within the larger multicast range of 184.108.40.206 to 220.127.116.11 (18.104.22.168/3). The specifics of the range you’ll need to configure depend on the application and your environment, but keep in mind that unlike unicast addresses, you don’t need assign one multicast address per client. However, each multicast client will additionally need a unique, unicast address.
Important: MADCAP is analogous to DHCP but for multicast addresses. You configure multicast scopes in the DHCP console. Remember the multicast address range.