These routing protocols use several algorithms to determine how to route and deliver packets. (For more information about these algorithms, see Chuck Semeria and Tom Maufer, "Introduction to IP Multicast Routing," http://www.3com.com/nsc/501303.html.)
Enter MADCAP
Whereas DHCP issues unicast addresses to clients, the Multicast Address Dynamic Client Allocation Protocol (MADCAP, formerly known as Multicast DHCP—MDHCP) issues multicast addresses. Clients (i.e., multicast applications and client computers) lease an address to join a multicast group and can renew or cancel the lease at any time. Before a client can lease an address, you must create a scope that offers clients a range of addresses from which to choose.
To create a scope in Win2K, open the Microsoft Management Console (MMC) DHCP snap-in, then select and right-click the DHCP server. Select New Multicast Scope to open the New Multicast Scope Wizard, which prompts you to enter a name, description, start address, and end address for the scope. MADCAP lets you use 239.0.0.0 to 239
.254.0.0 for your scope. RFC 2356 recommends that you keep the scope within the range 239.192.0.0 to 239.252.0.0. (For more information about RFC 2356, see "Related Reading.") Microsoft online Help recommends that you use one of the following ranges:
- 239.253.0.0 to 239.253.255.255
- 239.254.0.0 to 239.254.255.255
- 239.255.0.0 to 239.255.255.255
The wizard also prompts you to enter a Time to Live (TTL) variable, which defines how many routers traffic can pass through, and a lease duration. Win2K's MADCAP default for TTL is 32, and the default lease duration is 30 days. After you activate the scope, it appears in the left frame of the DHCP snap-in, and clients can request multicast addresses through application APIs.
By default, multicast scopes don't expire (i.e., the lifetime is Infinite), but you can set an expiration date and time after which the scope will disappear from the system. To change a scope's lifetime, right-click the scope in the DHCP snap-in, select Properties to open the Properties dialog box, go to the Lifetime tab, which Figure 4 shows, and define an expiration date and time.
Although you use the DHCP snap-in to configure MADCAP, the DHCP and MADCAP services function independently of one another. However, a client must receive its IP configuration before the server can issue a multicast address to the client.
Head of the Class
With Win2K putting so much emphasis on new multicasting-enabled applications, now is a good time to rethink your network's IP addresses. Consider the ways in which multicasting and Class D addresses might open up new possibilities.