High Class

Working with IP addresses above Class C

A majority of networks today run TCP/IP, so the odds are good that your network uses IP addresses that you probably inherited from a previous administrator. You might be tempted to simply accept these existing IP addresses. If and when you create new addresses, you might not think to look beyond Classes A, B, or C. However, you can use Class D IP addresses to increase your network's efficiency and benefit multicasting applications.

Unicasting vs. Multicasting and Broadcasting
Every Windows 2000 or Windows NT book you pick up will point out that you can use Class A, B, and C IP-address ranges on your network. But two more classes (i.e., Class D and Class E) offer values in the range beyond 223, which is the upper limit of Class C's first octet.

IP uses unicasting to communicate between addresses in Classes A, B, and C. This method entails a host sending a communication to one other host, as Figure 1, page 102, shows. Usable addresses in Classes D and E use multicasting, which entails a host sending a communication to multiple select hosts, as Figure 2, page 102, shows, or broadcasting, which entails a host sending a communication to all other hosts on the subnet, as Figure 3, page 102, shows. (Class A, B, and C addresses can also employ broadcasting, but not by default.)

Broadcasting and Class E
Broadcasting generates traffic on the Internet and should be used selectively. That said, if you want to send a broadcast to a subnet other than the local subnet, you can use 255 as the ending octet or octets. For example, to send a broadcast to the entire 207.16.99 network, use the address To send a broadcast to the Class B address 133.87, use the address

A Class E address' first octet can range in value from 240 to 255. With rare exceptions, these addresses are reserved for future use and experimentation. The primary exception is, which is used to send a LAN broadcast (often to a boot server).

Multicasting and Class D
Multicasting is more efficient than unicasting, and much more efficient than broadcasting, for delivering data to multiple recipients. For example, to send a slide presentation to 300 users on 300 hosts, unicasting sends the slide presentation 300 times. To perform the same task, multicasting sends the slide presentation once to one Class D address that represents a multicast group to which the 300 hosts subscribe. Thus, multicasting reduces the amount of network traffic and the amount of consumed bandwidth.

Any host in any location can subscribe to a multicast group; the subscribing host needn't be on the same subnet as the sending host. A host can subscribe to more than one multicast group. Each host in a group receives the same data as the other hosts in the group. (However, multicast packets might not arrive in the order in which a host sends them, so subscribing hosts might need to compile and sort multiple packets to reassemble the sent data.) Multicasting is the foundation for Multicast Backbone (MBone), videoconferencing, and similar multicast-enabled applications. (For more information about multicasting applications, see Tao Zhou, "Be Prepared for IP Multicasting Applications," May 1997.)

You can use Class D addresses for multicasting. A Class D address' first octet can range in value from 224 to 239. Keep in mind that most addresses in the 224 range are reserved for specific purposes; you shouldn't use these addresses. (Some Class A, B, and C addresses are also reserved. For more information about these addresses, see the sidebar "Other Special Addresses.") Among the reserved Class D addresses are the following:

  • (for multicasting to all hosts on a subnet)
  • (for multicasting to all routers on a subnet)
  • (for multicasting to all Distance Vector Multicast Routing Protocol—DVMRP—routers)
  • (for multicasting to all Multicast Open Shortest Path First—MOSPF— routers)
  • (for multicasting to all Routing Information Protocol version 2— RIP2—routers)
  • (for multicasting to all Interior Gateway Routing Protocol— IGRP—routers)
  • Addresses in the range from to (for use by MSNBC)
  • Addresses in the range from to (for use by Dow Jones)

You can use multicasting within a private organization to communicate with a selected group of users anywhere. The Internet Engineering Task Force (IETF) and the Internet Assigned Numbers Authority (IANA) recommend that you use an address in the to range for one organization. If you want to multicast over the Internet, you must obtain a multicast address from the IANA.

Multicast Routing
Multicast routing involves sending messages to or among multiple routers for the purpose of updating routing tables. A multicast router uses the Internet Group Management Protocol (IGMP), which defines membership within a multicast group, to examine a multicast packet's header and determine whether to send data to local recipients. You can use Mcast, a diagnostic utility in the Microsoft Windows 2000 Resource Kit, to determine which routers on your network are enabled for IP multicasting. Multicast routers communicate with one another using one or more of the following protocols:

  • MOSPF—IETF Request for Comments (RFC) 1584 defines MOSPF. This protocol is one of the best examples of multicast router communication.
  • DVMRP—RFC 1075 defines this protocol, which uses Link State Advertisements, so each router can multi-cast available links within the network.
  • Protocol Independent Multicast-Sparse Mode (PIM-SM)—RFC 2362 defines this protocol, which replaces RFC 2117. PIM-SM lets unicast routers support multicasting. You typically use PIM-SM in conjunction with IGRP.

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.)

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 to 239 .254.0.0 for your scope. RFC 2356 recommends that you keep the scope within the range to (For more information about RFC 2356, see "Related Reading.") Microsoft online Help recommends that you use one of the following ranges:

  • to
  • to
  • to

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.

Related Articles in Previous Issues
You can obtain the following articles from Windows 2000 Magazine's Web site at http://www.win2000mag.com.

"Win2K's Premier IP Multicast Functionality," November 2000, InstantDoc ID 15711
"Be Prepared for IP Multicasting Applications," May 1997, InstantDoc ID 39

TCP/IP, Signature edition (McGraw-Hill, 1998)

"Introduction to IP Multicast Routing"
"Multicast Routing"
RFC 2365
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