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Unformatted text preview: every time one of the 20 sends a UDP broadcast, the other 30 hosts have to
process the broadcast, all the way up through the UDP layer, before the UDP datagram is
discarded. The UDP datagram is discarded by these 30 hosts because the destination port
number is not in use.
The intent of multicasting is to reduce this load on hosts with no interest in the
application. With multicasting a host specifically joins one or more multicast groups. If
possible, the interface card is told which multicast groups the host belongs to, and only
those multicast frames are received. 12.2 Broadcasting
In Figure 3.9 we showed four different forms of IP broadcast addresses. We now describe
them in more detail.
The limited broadcast address is 255.255.255.255. This can be used as the destination
address of an IP datagram during the host configuration process, when the host might not
know its subnet mask or even its IP address.
A datagram destined for the limited broadcast address is never forwarded by a router
under any circumstance. It only appears on the local cable.
An unanswered question is: if a host is multihomed and a process sends a datagram to the
limited broadcast address, should the datagram be sent out each connected interface that
supports broadcasting? If not, an application that wants to broadcast out all interfaces
must determine all the interfaces on the host that support broadcasting, and send a copy
out each interface.
Most BSD systems treat 255.255.255.255 as an alias for the broadcast address of the first
interface that was configured, and don't provide any way to send a datagram out all
attached, broadcast-capable interfaces. Indeed, two applications that send UDP datagrams
out every interface are routed (Section 10.3) and rwhod (the server for the BSD rwho
client). Both of these applications go through a similar start-up procedure to determine all
the interfaces on the host, and which ones are capable of broadcasting. The net-directed
broadcast address corresponding to that interface is...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12