Differences between unicast, broadcast and multicast of a packet

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Differences between unicast, broadcast and multicast of a packet

Post  Code Maestro on Thu Nov 08, 2012 4:17 pm

Explain the differences between unicast, broadcast and multicast of a packet. Be sure to
mention the IP addresses used for each.
For a multicast system explain how a multicast router determines which multicast
groups have members among the hosts on its directly connected networks (the networks
directly attached to the router).



Unicast: Sent from one source to one receiver. The source and the receiver may be on the
same network or on different networks. If classfull addressing is used the IP addresses used
for each host (can be either a source or a receiver) will be part of a Class A, B or C address.
For classfull or CIDR addressing the unicast addresses will fall between 1.0.0.0 and
223.255.255.255.

Broadcast. Sent from one source to every node in a network. The IP broadcast address has
two forms. For use inside the network, the limited broadcast address, is composed of all 1’s
for both the netid and the hostid. This broadcasts from one node in the network to all nodes
in the network. For use from outside the network the broadcast address is composed of the
netid of the network, with all 1’s for the hostid. This address is often disabled to prevent
denial of service attacks.

Multicast: Sent from one or more sources to one of more receivers. Only single source
multicast from one source to many receivers was discussed in class. For single source
multicast a source can send a packet to a group of receivers. The receivers may be part of the
local network and may also be part of any number of other networks that support multicast.
The addresses used for multicast are the Class D addresses between 224.0.0.0 and
239.255.255.255.

A multicast router (different process from a unicast router) determines which multicast
groups have members among the hosts on its directly connected networks in two ways.
When a host on a network directly attached to the multicast router requests to become a
member of a multicast group the multicast router will receive a copy of the IGMP
membership report message that the joining host sent the multicast address of the group it
wished to join. If there are other members in the group the host requested to join that reside
on the networks directly connected to the multicast router then the name of the joining host
will be added to the members of the group. If there are no members in the group on any
networks directly connected to the multicast router the new group will be added to the list of
groups the multicast router is keeping track of. The multicast group will then be recorded as
having a single member among the hosts on networks directly attached to the multicast
router. To maintain the lists of members of active multicast groups (those with members on
networks directly attached to the multicast router) the router will periodically send a IGMP
membership query to all hosts on directly attached networks. This query will request that all
members of each active group respond to the multicast servers query indicating which active
multicast groups they belong to. When a host receives a membership query it prepares to
send a response (a membership report) for each multicast group of which it is a member.
Each host will set a separate timer for each reply it wishes to send. While waiting for the time
associated with multicast group A’s reply to expire the host watches for other responses from
multicast group A. If it sees such a response it will drop its own response and terminate the
timer. If it sees no response before its own timer expires it will then send its own response.
This procedure limits the number of replies sent and reduces the load on the network. The
multicast router only needs to know if it needs to continue to maintain the multicast address
as a active group. To do this it needs to receive only one response from attached networks.
To leave the group the host will send a leave message to the multicast router.

Code Maestro

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