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Unformatted text preview: are three kinds of IP addresses: unicast, broadcast,
and multicast. In this chapter we discuss broadcasting and multicasting in more detail.
Broadcasting and multicasting only apply to UDP, where it makes sense for an
application to send a single message to multiple recipients. TCP is a connection-oriented
protocol that implies a connection between two hosts (specified by IP addresses) and one
process on each host (specified by port numbers).
Consider a set of hosts on a shared network such as an Ethernet. Each Ethernet frame
contains the source and destination Ethernet addresses (48-bit values). Normally each
Ethernet frame is destined for a single host. The destination address specifies a single
interface-called a unicast. In this way communication between any two hosts doesn't
bother any of the remaining hosts on the cable (except for possible contention for the
There are times, however, when a host wants to send a frame to every other host on the
cable-called a broadcast. We saw this with ARP and RARP. Multicasting fits between
unicasting and broadcasting: the frame should be delivered to a set of hosts that belong to
a multicast group.
To understand broadcasting and multicasting we need to understand that filtering takes
place on each host, each time a frame passes by on the cable. Figure 12.1 shows a picture
First, the interface card sees every frame that passes by on the cable and makes a decision
whether to receive the frame and deliver it to the device driver. Normally the interface
card receives only those frames whose destination address is either the hardware address
of the interface or the broadcast address. Additionally, most interfaces can be placed into
a promiscuous mode whereby they receive a copy of every frame. This mode is used by
tcpdump, for example. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/broadcas.htm (1 of 12) [12/09/2001 14.47.00] Chapter 12. Broadcasting and Multicasting Figure 12.1 Filtering that takes place up the protocol...
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- Spring '12