Using multicast scopes

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Using multicast scopes

Multicast scopes are supported through the use of Multicast Address Dynamic Client Allocation Protocol (MADCAP), a proposed standard protocol for performing multicast address allocation. The MADCAP protocol describes how multicast address allocation (or MADCAP) servers can dynamically provide IP addresses to other computers (MADCAP clients) on your network.

Typically, a MADCAP client might also be a multicast server (MCS) used to support IP multicasting. An MCS manages the shared or group use of the allocated multicast IP address and streams data traffic to members that share the use of the specified group address.

Once an MCS is configured and allocated a group IP address to use, any multicast clients that have registered their membership with the MCS can receive streams sent to this address. By registering with the MCS, clients can participate efficiently in the stream process, such as for real-time video or audio network transmissions. The MCS also manages the multicast group list, updating its membership and status so that multicast traffic is received by all current members.

Background on multicast addresses

Ordinarily, you use DHCP scopes to provide client configurations by allocating ranges of IP addresses from the standard address classes, Class A, B, or C. By using DHCP scopes you can assign IP addresses from the ranges provided by these classes for your DHCP clients to be configured to use unicast (or point-to-point) directed communication between other TCP/IP networked computers.

The multicast address range uses an additional address class, Class D, that includes IP addresses that range from to for use in IP multicasting. Addresses in this class are used for multicasting only and not for regular DHCP scopes.

In all TCP/IP networks, each host is required to first be configured with its own IP address, taken from one of the standard address classes. You must assign this required unicast IP address before you can configure a host to support and use secondary IP addresses, such as a multicast IP address.

Another difference between unicast and multicast addresses, is that a group of TCP/IP host computers are intended to be able to share the use of a multicast IP address. This is not normally the case with unicast IP addresses, which are intended to be assigned individually to only one configured host and not shared with other hosts.

When the destination address for an IP datagram is an IP multicast address, the datagram is forwarded to all members of a multicast group, which is a set of zero or more hosts identified by the address. The membership of a multicast group is dynamic in that individual hosts can join or leave the group at any time.

Membership and use of multicast groups is unrestricted and can be compared to membership and use of a group e-mail address: Group membership can be any size, and hosts can be members of many multicast groups.

You can permanently reserve multicast group addresses or temporarily assign and use them as needed on your network. For a permanent group IP address to be reserved for Internet use, you must register it with the Internet Assigned Numbers Authority (IANA).

For multicast IP addresses not permanently reserved with the IANA, all Class D addresses that remain unreserved can then be used dynamically to assign and form temporary multicast groups. These temporary groups can exist as long as one or more hosts on the network are configured with the group address and actively share its use.

For more information, see Understanding Multicasting or Internet Group Management Protocol (IGMP).

Determining the ranges to use for multicast scopes

When deciding the IP address ranges to use for multicast scopes on your MADCAP server, there are two overall best practices recommended by the Multicast Allocation (MALLOC) working group, an IETF team of industry volunteers who help establish multicast address allocation standards. These practices include:

  • Administrative scoping  This is most useful when you are using multicast IP addresses privately in your own network. It is similar to private IP addressing, as used currently in the unicast IP address spaces (such as the use of the network address space). It is fully discussed in RFC 2365, "Administratively Scoped IP Multicast."

    With administratively scoped multicast IP, the range most recommended that you begin with is the range. This range is known as the IPv4 Organization Local Scope and has a subnet mask of (14-bits in length). It is intended for use by an entire organization setting multicast scopes privately for its own internal or organizational use. Starting with this address, you can create a considerable number of addresses--up to 218, or 262,144 group addresses--for use in all subnets within your organization's network.

    For more information, refer directly to the RFC, which can be obtained at the Request for Comments Web site.

  • Global scoping  This practice is most useful when you are using multicast group IP addresses in a public network address space, particularly the Internet.

    Because most organizations requiring one or more public addresses likely have already been assigned some public unicast IP addresses, a subnet allocation scheme has been proposed. For MADCAP, the range of the Class D address space is recommended for use as a global scope range.

    When the range is used, the allocation of global addresses works in the following way:

    1. A network registry, such as the Internet Assigned Numbers Authority (IANA), allocates and reserves the first 8 bits of the range (for example, the "233" portion of this range).

    2. The next 16 bits (the two middle octet numbers of addresses in this range) are based on a previously assigned Autonomous System (AS) number. This number is recorded with the applicable Internet Assigned Numbers Authority (IANA) registry for your region.

      If you already register your IP addresses with a regional network registry, you might know your AS number. You can also look it up easily using the Whois database system on the Internet. If you are obtaining IP addresses through an Internet service provider (ISP), you might contact them to find out what your AS number is.

      AS numbers are allocated to the regional registries by the IANA. If you or your ISP are located in the United States, you can apply directly to the American Registry for Internet Numbers (ARIN) to obtain an AS number. For more information, see the ARIN Web site.

      For other regions, AS numbers can also be obtained from appropriate regional registries. Other regional registries include the following:

ARIN or <> for the Americas, Caribbean, and Africa.

RIPE-NCC <> for Europe.

AP-NIC <> for the Asia-Pacific region.

  1. The last 8 bits in the address are local use bits.

    These provide the IP address range from which to configure any multicast scopes for group addresses you want to use publicly on the Internet. With this global scoping system in use, a subnet mask of should be applied. This provides each organization with an assigned AS number of up to 255 multicast group addresses for use on the Internet.

    For more information on either the practice of global scoping for multicast address allocation or the use and allocation of AS numbers, use the IETF Web site. For global scoping, refer directly to the current available draft entitled "Static Allocation in 233/8," and for more information on AS numbering, refer to RFC 1930, "Guidelines for creation, selection, and registration of an Autonomous System (AS)."


The Windows Server 2003 DHCP service supports both the DHCP and MADCAP protocols. These protocols function separately and are not dependent on each other. For example, a DHCP client might or might not be a MADCAP client and a MADCAP client might or might not be a DHCP client.

It is also worth mentioning that the DHCP Server service can be used to deploy MADCAP servers independent of how DHCP servers are used on your network. For example, to install Windows Server 2003 DHCP for MADCAP service only:

  1. Create multicast scopes.

  2. Do not create other scopes or superscopes.

Only where other scopes or superscopes are configured does the server computer also function as a DHCP server.

For more information about MADCAP, see Multicast address allocation.


  • Multicast scopes and MADCAP only provide a mechanism for dynamically allocating IP address configuration for multicast-ranged IP addresses. Other network configuration details are normally required to enable multicasting for your deployment needs. For more information, see Checklist: Installing IP multicast video conferencing.