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Gemini G3 - User Manual
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4.7.3.6.1
IP Broadcast/Multicast Overview
When an IP packet needs to reach more than one unit, the destination address can be set to either a
broadcast address or a multicast address.
4.7.3.6.1.1
Broadcasts
Figure 32 - Broadcast Window Detail
There are two types of IP broadcast addresses:
•
Directed broadcast
A directed broadcast address is an IP address where the host portion is all ones (for instance
172.30.1.255 is the directed broadcast address for the network 172.30.1.0/24, 172.30.1.207 is
the directed broadcast address for the network 172.30.1.192/28).
•
Limited broadcast
The limited broadcast address is 255.255.255.255.
Note:
Routing equipment (to prevent broadcast storms) do not by default forward limited
broadcast packets (255.255.255.255). On the other hand, directed broadcast packets
are by default forwarded because these packets are routable like any other unicast
packets.
4.7.3.6.1.1.1
Directed Broadcast
Each interface of a unit has its own IP address and netmask. From the IP address and netmask, it is
easy to calculate the broadcast address associated to the interface. For instance, if the Ethernet inter-
face address of a GeminiG3 radiomodem is 172.30.1.1/24 and the RF interface address is 10.0.1.2/24,
then the broadcast address of the Ethernet interface is 172.30.1.255 and the broadcast address of the
RF interface is 10.0.1.255.
The “
Directed Broadcast
” option buttons let the user select whether the unit must forward
(or not)
di-
rected broadcast
packets. Upon reception of a
directed broadcast
packet, the unit takes the following
actions:
If the directed broadcast address matches with one of the unit’s interface broadcast addresses:
•
Keep a copy for itself (pass to internal applications, if any).
•
If directed broadcast packets can be forwarded (Directed Broadcast is enabled):
Forwards the packet according to the routing table.
•
If directed broadcast packets cannot be forwarded (Directed Broadcast is disabled):
Silently discards the packet.