Training
Certifications
Books
Special Offers
Community




 
Microsoft® Windows® Server 2003 TCP/IP Protocols and Services Technical Reference
Author Joseph Davies and Thomas Lee
Pages 768
Disk 1 Companion CD(s)
Level Int/Adv
Published 02/26/2003
ISBN 9780735612914
Price $49.99
To see this book's discounted price, select a reseller below.
 

More Information

About the Book
Table of Contents
Sample Chapter
Index
Related Series
Related Books
About the Author

Support: Book & CD

Rate this book
Barnes Noble Amazon Quantum Books

 


Chapter 6: Internet Protocol (IP) Addressing


IP Broadcast Addresses

IP broadcast addresses are used for single-packet one-to-everyone delivery. A sending host addresses the IP packet using a broadcast address and every node on the sending node's network segment receives and processes the packet. IP broadcast addresses can be used only as the destination IP address.

There are four different types of IP broadcast addresses. For each type, the broadcast IP packet is addressed at the Network Interface Layer using the network technology's broadcast address. For example, for Ethernet and Token Ring networks, all IP broadcasts are sent using the Ethernet and Token Ring broadcast address 0xFF-FF-FF-FF-FF-FF.

Network Broadcast

The IP network broadcast address is the address formed by setting all the host bits to 1 for a classful address. An example of a network broadcast address for the classful network ID 131.107.0.0/16 is 131.107.255.255. Network broadcasts are used to send packets to all hosts of a classful network, which listen for and process packets addressed to the network broadcast address. IP routers do not forward network broadcast packets.

Subnet Broadcast

The IP subnet broadcast address is the address formed by setting all the host bits to 1 for a nonclassful address. An example of a network broadcast address for the nonclassful network ID 131.107.26.0/24 is 131.107.26.255. Subnet broadcasts are used to send packets to all hosts of a subnetted, supernetted, or otherwise nonclassful network. All hosts of a nonclassful network listen for and process packets addressed to the subnet broadcast address. IP routers do not forward subnet broadcast packets.

For a classful network, there is no subnet broadcast address, only a network broadcast address. For a nonclassful network, there is no network broadcast address, only a subnet broadcast address.

All-Subnets-Directed Broadcast

The IP all-subnets-directed broadcast address is the address formed by setting all the original classful network ID host bits to 1 for a nonclassful network. A packet addressed to the all-subnets-directed broadcast is intended to reach all hosts on all of the subnets of a subnetted class-based network ID. An example of an all-subnets-directed broadcast address for the subnetted network ID 131.107.26.0/24 is 131.107.255.255. The all-subnets-directed broadcast is the network broadcast address of the original classful network ID.

All hosts of a nonclassful network listen for and process packets addressed to the all-subnets-directed broadcast address. RFC 922 required IP routers to forward all-subnets-directed broadcast packets to all subnets of the original classful network ID implied in the address. However, this forwarding was not widely implemented.

With the advent of classless network IDs, the all-subnets-directed broadcast address is no longer relevant. According to RFC 1812, the use of the all-subnets-directed broadcast has been deprecated.

Notice how the all-subnets-directed address is the same as the subnet broadcast for the all-ones subnet. For example, the 8-bit subnetting of the class B network ID 157.54.0.0 produces the subnets {157.54.0.0/24, 157.54.1.0/24 . . . 157.54.254.0/24, 157.54.255.0/24}. For the last subnet, 157.54.255.0/24, the subnet broadcast is 157.54.255.255, which is the same as the all-subnets-directed broadcast address of 157.54.255.255. This address conflict is not an issue for routers that do not forward all-subnets-directed broadcast traffic.

Limited Broadcast

The limited broadcast address is the address formed by setting all 32 bits of the IP address to 1 (255.255.255.255). The limited broadcast address is used when an IP node must perform a one-to-everyone delivery on the local network but the network ID is unknown.

The limited broadcast address is typically used only by nodes during an automated configuration process such as Boot Protocol (BOOTP) or DHCP. For example, with DHCP, a DHCP client must use the limited broadcast address for all traffic sent until the DHCP server acknowledges the IP address lease.

All hosts, classful or nonclassful, listen for and process packets addressed to the limited broadcast address. Although it appears that the limited broadcast address is addressed to all nodes on all networks, it appears only on the local network and is never forwarded by routers. The limited broadcast packet is limited to the local network segment.

The following registry setting controls the address of the limited broadcast address:

UseZeroBroadcast

Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters \InterfaceGUID
Value type: REG_DWORD
Valid range: 0 - 1
Default: 0
Present by default: Yes

UseZeroBroadcast determines whether the limited broadcast is 0.0.0.0 (when set to 1) or 255.255.255.255 (when set to 0). By default, UseZeroBroadcast is set to 0. Some implementations of TCP/IP, such as those derived from UNIX, use 0.0.0.0 as their limited broadcast address. On the same subnet, all nodes should be using the same limited broadcast address.

IP Multicast Addresses

IP multicast addresses are used for single-packet one-to-many delivery. A sending host addresses the IP packet using an IP multicast address; every node on the sending node's internetwork that is listening for the multicast traffic receives and processes the packet. Unlike broadcast packets, routers forward IP multicast packets and only the hosts listening for the IP multicast traffic are disturbed. IP multicast addresses can be used only as the destination IP address.

As RFC 1112 describes, the set of hosts listening for the traffic of a specific IP multicast address is called a host group. Host group members can be located anywhere on the IP internetwork. They also can join and leave the host group at any time. For routers to forward IP multicast traffic to host group members, the routers must be aware of where the members of a multicast group are located. For more information on how hosts and routers facilitate the forwarding of IP multicast traffic, see Chapter 9, "Internet Group Management Protocol (IGMP)."

Multicast IP addresses are in the class D range. Multicast IP addresses range from 224.0.0.0 through 239.255.255.255 (224.0.0.0/4). Multicast IP addresses in the range 224.0.0.0 through 224.0.0.255 (224.0.0.0/24) are reserved for local subnet traffic. Table 6-16 lists some of the reserved IP addresses in this range used by the Windows Server 2003 family. For a complete list, see http://www.iana.org/assignments/multicast-addresses.

Table 6-16. Reserved Local Subnet IP Multicast Addresses

Multicast IP AddressPurpose
224.0.0.1The all-hosts multicast address, designed to reach all hosts on a subnet
224.0.0.2The all-routers multicast address, designed to reach all routers on a subnet
224.0.0.5The AllSPFRouters address, designed to reach all OSPF routers on a subnet
224.0.0.6The AllDRRouters address, designed to reach all OSPF designated routers on a subnet
224.0.0.9The RIP version 2 multicast address, designed to reach all RIP version 2 routers on a subnet

Mapping IP Multicast Addresses to MAC Addresses

To fulfill the promise of IP multicast traffic—where a single IP datagram is processed only by the host group members—IP multicast traffic must be mapped to a corresponding MAC-level multicast address. The corresponding MAC-level multicast becomes an interesting address to the network interface card (NIC), and all traffic addressed to that interesting address with a valid frame check sequence is passed up through a hardware interrupt to the operating system.

Ethernet and Fiber Distributed Data Interface

To denote a MAC-level multicast address, Ethernet and Fiber Distributed Data Interface (FDDI) network adapters set the Individual/Group (I/G) bit, the low-order bit of the first byte of the destination MAC address, to 1. For IP multicast addressing, the range of multicast MAC addresses is 0x01-00-5E-00-00-00 to 0x01-00-5E-7F-FF-FF. The high-order 25 bits are set to 0000001 00000000 01011110 0. The low-order 23 bits are available for use by IP multicast addresses.

To map an IP multicast address to an Ethernet or FDDI MAC-level multicast address, the low-order 23 bits of the IP multicast address are copied to the low-order 23 bits in the Ethernet multicast address as Figure 6-11 shows.

Click to view graphic
Click to view graphic

Figure 6-11. The mapping of IP multicast addresses to Ethernet and FDDI MAC addresses.

In the high-order 9 bits of the IP multicast address, the first 4 bits are set to 1110; the next 5 bits are variable. These 5 bits do not map to the corresponding Ethernet and FDDI multicast address. Therefore, up to 32 different IP multicast addresses can map to the same Ethernet and FDDI MAC-level multicast address. IP multicast packets received that do not correspond to a multicast address registered by an application or another protocol are silently discarded.

A node registers interest in a specific multicast group by informing the NIC to listen for another interesting destination address for incoming frames. In the Windows Server 2003 family, this is done through the NDISRequest( ) function. For example, by default TCP/IP for the Windows Server 2003 family listens for all multicast traffic sent to the all-hosts multicast address 224.0.0.1. Therefore, TCP/IP informs the NIC through NDIS to pass up frames with the destination MAC address of 0x01-00-5E-00-00-01.

Token Ring

As RFC 1469 describes, Token Ring can support the same type of multicast IP-address-to-MAC mapping as Ethernet and FDDI. However, because of the hardware limitations of most Token Ring network adapters, typically all IP multicast addresses are mapped to the single Token Ring functional address of 0xC0-00-00-04-00-00.

TCP/IP for the Windows Server 2003 family multicast behavior for Token Ring is controlled by the following registry setting:

TrFunctionalMcastAddress

Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
Value type: REG_DWORD
Valid range: 0 - 1
Default: 1
Present by default: No

TrFunctionalMcastAddress either enables (when set to 1) or disables (when set to 0) the use of the Token Ring functional address of 0xC0-00-00-04-00-00 for all IP multicast traffic sent on Token Ring adapters. If disabled, IP multicast traffic is sent using the MAC-level broadcast address 0xFF-FF-FF-FF-FF-FF. This setting is enabled by default.

Summary

IP addresses can be unicast, broadcast, or multicast. For unicast addresses, subnetting techniques allow a network ID to be allocated, in an efficient manner, to the subnets of an IP internetwork. Internet authorities have defined public addresses that are reachable on the Internet and private addresses that are designed for use on private intranets not directly connected to the Internet. IP broadcast addresses are used to send IP datagrams to all the nodes on a physical or logical subnet. IP multicast addresses are used to send IP datagrams to all members of a multicast host group.


Previous



Last Updated: February 9, 2003
Top of Page