TCP/IP Tutorial and Technical Overview - IBM Redbooks

outers. Without this facility, a separate BOOTP server is required on each
subnet (refer to 3.6, “Bootstrap Protocol (BOOTP)” on page 125).
► Network-directed broadcast address: This is used in an unsubnetted
environment. The network number is a valid network number and the host
number is all ones (for example, 128.2.255.255). This address refers to all
hosts on the specified network. Routers should forward these broadcast
messages. This is used in ARP requests (refer to 3.4, “Address Resolution
Protocol (ARP)” on page 119) on unsubnetted networks.
► Subnet-directed broadcast address: If the network number is a valid network
number, the subnet number is a valid subnet number, and the host number is
all ones, the address refers to all hosts on the specified subnet. Because the
sender's subnet and the target subnet might have a different subnet mask,
the sender must somehow determine the subnet mask in use at the target.
The broadcast is performed by the router that receives the datagram into the
subnet.
► All-subnets-directed broadcast address: If the network number is a valid
network number, the network is subnetted, and the local part is all ones (for
example, 128.2.255.255), the address refers to all hosts on all subnets in the
specified network. In principle, routers can propagate broadcasts for all
subnets but are not required to do so. In practice, they do not. There are very
few circumstances where such a broadcast is desirable. If misconfigured, it
can lead to problems. Consider the misconfigured host 9.180.214.114 in a
subnetted Class A network. If the device was configured with the address
9.255.255.255 as a local broadcast address instead of 9.180.214.255, all of
the routers in the network will forward the request to all clients.
If routers do respect all-subnets-directed broadcast address, they use an
algorithm called reverse path forwarding to prevent the broadcast messages
from multiplying out of control. See RFC 922 for more details about this
algorithm.
Multicasting
If an IP datagram is broadcast to a subnet, it is received by every host on the
subnet. Each host processes the packet to determine if the target protocol is
active. If it is not active, the IP datagram is discarded. Multicasting avoids this by
selecting destination groups.
Each group is represented by a Class D IP address. For each multicast address,
a set of zero or more hosts are listening for packets addressed to the address.
This set of hosts is called the host group. Packets sent to a multicast address are
forwarded only to the members of the corresponding host group. Multicast
enables one-to-many connections (refer to Chapter 6, “IP multicast” on
page 237).
Chapter 3. Internetworking protocols 85