Learn more: download the IP Routing: Multicast datasheet. Skip to Content. A Microsoft Company. What is multicast IP routing? A Multicast Group Membership Discovery protocol is used by receiving hosts to advertise their group membership to a local multicast router, enabling them to join and leave multicast groups. A Multicast Routing Protocol is used to communicate between multicast routers and enables them to calculate the multicast distribution tree of receiving hosts.
When the network card picks up a packet which has a destination MAC that matches any of the multicast MAC addresses, it will pass it to the upper layers for further processing. Ethernet uses the low-order bit of the high-order octet to distinguish conventional unicast addresses from multicast addresses.
To understand this, we need to analyse the destination MAC address of a unicast and multicast packet, so you can see what we are talking about:. The following picture is an example of my workstation When my gateway receives the packet, it knows it's a unicast packet as explained in the above picture.
Let's now have a look at the MAC address of a multicast packet. Keep in mind, a multicast packet is not directed to one host but a number of hosts, so the destination MAC address will not match the unique MAC address of any computer, but the computers which are part of the multicast group will recognise the destination MAC address and accept it for processing.
The following multicast packet was sent from my NetWare server. Notice the destination MAC address it's a multicast :. Analysis of a multicast destination MAC address:.
So now you should be able to understand how computers can differentiate between a normal or unicast packet and a multicast packet. Again, the destination MAC address E is not the MAC address of a particular host-computer but the MAC address that can be recognised by computers that are part of the multicast group.
I should also note that you will never find a source address that is a multicast MAC address, the source address will always be a real one, to identify which computer the packet came from. This Rule is covered in the last section of this page, but you don't need to know it now in order to understand Hardware multicasting. The IP Multicast is the second part of multicasting which, combined with the hardware multicasting, gives us a multicasting model that works for our Ethernet network.
If hardware multicasting fails to work, then the packet will never arrive at the network layer upon which IP multicasting is based, so the whole model fails. Once Layer 2 Datalink picks the multicast packet from the network because it recognises it, as the destination MAC address is a multicast it will strip the MAC addresses off and send the rest to the above layer, which is the Network Layer.
At that point, the Network Layer needs to be able to understand it's dealing with a multicast, so the IP address is set in a way that allows the computer to see it as a multicast datagram. A host may send multicast datagrams to a multicast group without being a member. Multicasts are used a lot between routers so they can discover each other on an IP network.
The OSPF router must send this "hello" packet to an assigned multicast address, which is The picture below is a screenshot from my packet sniffer, it shows a multicast packet which was sent from my NetWare server, notice the destination IP address:. The screenshot above shows the packet which was captured, it's simply displaying a quick summary of what was caught.
Could you please elaborate on that particular topic when you have a moment? Thank you in advance. Hi You will find quite a lot of multicast traffic on a network as it is used for device discovery. I will try to find time to talk about monitoring tools that may help. Have adjusted the TTL accordingly with no luck. Someone mentioned Comcast could be blocking the traffic. Thanks, Tom. Here is a quote from stackoverflow. There are some techniques to work around this DVMRP, MOSPF and others but they all require that you can configure all the routers between your server and the clients or create a tunnel.
There are backbone networks Abilene, Mbone with multicast support, but those are of most interest for universities and such. Unfortunately you need point-to-point communication.
But you are in good company, internet, radio and TV all do point-to-point, transmitting the same data numerous times. Quite a waste of bandwidth. Easiest way is to configure all switches locally to pass through UDP on level 2. Then the TTL will decide whether the multicsat will be forwarded or not depending how many hops the packet did already have passed.
Level 3 switches do sniff the join group address Some vendors call this UDP spoofing also they actively manage the multicast groups that was detected. The join group will go along with a group number which is equal to the IP port used. Pls help me. How do i achieve this? What equipment do i want? Where can i buy this equipments. Hi I received a message that my yahoo account was tried for an unauthorised access.
The message contains details of ip address — In this case, can an individual be trying to hack my email account. Is there any way to find who tried to hack? You response will be of a great help.
Many thanks in advance. Your email address will not be published. Skip to content What is IP Multicasting?
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