Broadcast and multicast mac addresses
Network layer protocols such as IPv4 also support a form of broadcast that allows the same packet to be sent to every system in a logical network in IPv4 this consists of the IP network ID and an all 1's host number. Multicast is the term used to describe communication where a piece of information is sent from one or more points to a set of other points.
In this case there is may be one or more senders, and the information is distributed to a set of receivers theer may be no receivers, or any other number of receivers. One example of an application which may use multicast is a video server sending out networked TV channels. Simultaneous delivery of high quality video to each of a large number of delivery platforms will exhaust the capability of even a high bandwidth network with a powerful video clip server.
Solved: understanding of unicast, multicast and - Cisco Community
This poses a major salability issue for applications which required sustained high bandwidth. One way to significantly ease scaling to larger groups of clients is to employ multicast networking.
Multicasting is the networking technique of delivering the same packet simultaneously to a group of clients. IP multicast provides dynamic many-to-many connectivity between a set of senders at least 1 and a group of receivers. The format of IP multicast packets is identical to that of unicast packets and is distinguished only by the use of a special class of destination address class D IPv4 address which denotes a specific multicast group.
Let me show you an example of a MAC address:.
Multicast address
Above you see an example of a MAC address. In the first octet, bit 0 has been reserved for broadcast or multicast traffic. When we have unicast traffic this bit will be set to 0. For broadcast or multicast traffic this bit will be set to 1.
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What about layer 2? What MAC addresses do we use for multicast traffic? For layer 2 we also have a reserved prefix to use for multicast traffic. The bit MAC address prefix E is reserved for layer 2 multicast. Unfortunately only half of the MAC addresses in this bit prefix can be used for multicast, this means we only have 23 bits of MAC address space to use for multicast.
Multicast IP Address to MAC address mapping
As you can see the first 3 octets are E. This is the reserved range. I just told you that only half of this bit space is available to us which means that only 23 bits can be used. Why can we only use 23 bits? Why does this matter? Let me show you:. Above you see an IP address which has 32 bits.
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A multicast IP address also has 32 bits but the first 4 bits are always the same because we use the This means that each multicast IP address has 28 unique bits. Now if we want to map our 28 bit multicast IP address to our 23 bit MAC address we have a problem…we miss 5 bits of mapping information: We miss 5 bits of mapping information: This can cause some problems in our networks.
Does the switch have a database of all possible multicast MAC addresses it references for each frame? No, that would be inefficient.
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- Unicast, multicast, and broadcast addresses;
How this is done efficiently is there is one specific bit in a Ethernet MAC address that signifies if the frame is unicast or multicast. The switch need only look at this one bit to make the distinction. The IEEE has specified that the most significant bit of the most significant byte be used for this purpose. If its a 1, that means multicast, 0 means unicast. The most significant byte is the left most byte in the address, and the most significant bit is the right most bit of the byte this is counter intuitive to most binary implementations where the left most bit usually labeled most significant.
Each of the bolded numbers represents a 1 or 0 present in the most significant bit of the most significant byte.