Multicast Addresses - Routing | Computer Networks - Computer Science Engineering (CSE) PDF Download

MulticastT Addresses
IP has a sub range of its address space reserved for multicast addresses. In IPv4, these addresses are assigned in the class D address space, and IPv6 also has a portion of its address space (see Table 4.11) reserved for multicast group addresses. Some sub ranges of the multicast ranges are reserved for intra domain multicast, so they can be reused independently by different domains.

Thus, there are 28 bits of possible multicast addresses in IPv4 when we ignore the prefix sharedby all multicast addresses. This presents a problem when attempting to take advantage of hardware multicasting on a LAN. Let’s take the case of Ethernet. Ethernet multicast addresses have only 23 bits when we ignore their shared prefix. In other words, to take advantage of Ethernet multicasting, IP has to map 28-bit IP multicast addresses into 23-bit Ethernet multicast addresses. This is implemented by taking the low-order 23 bits of any IP multicast address to use as its Ethernet multicast address, and ignoring the high-order 5 bits. Thus, 32 (25) IP addresses map into each one of the Ethernet addresses.

When a host on an Ethernet joins an IP multicast group, it configures its Ethernet interface to receive any packets with the corresponding Ethernet multicast address. Unfortunately, this causes the receiving host to receive not only the multicast traffic it desired, but also traffic sent to any of the other 31 IP multicast groups that map to the same Ethernet address, if they are routed to that Ethernet. Therefore, IP at the receiving host must examine the IP header of any multicast packet to determine whether the packet really belongs to the desired group. In summary, the mismatch of multicast address sizes means that multicast traffic may place a burden on hosts that are not even interested in the group to which the traffic was sent. Fortunately, in some switched networks (suchas switched Ethernet) this problem can be mitigated by schemes wherein the switches recognize unwanted packets and discard them.

The document Multicast Addresses - Routing | Computer Networks - Computer Science Engineering (CSE) is a part of the Computer Science Engineering (CSE) Course Computer Networks.
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FAQs on Multicast Addresses - Routing - Computer Networks - Computer Science Engineering (CSE)

1. What are multicast addresses used for in routing?
Ans. Multicast addresses are used in routing to efficiently deliver data packets to a group of recipients. Instead of sending a separate copy of the data to each recipient, multicast addresses allow the sender to send a single copy that is then replicated by routers to reach all members of the multicast group.
2. How are multicast addresses different from unicast and broadcast addresses?
Ans. Multicast addresses differ from unicast addresses as they represent a group of devices rather than a single destination. Unicast addresses are used for one-to-one communication, while multicast addresses enable one-to-many communication. Broadcast addresses, on the other hand, are used for one-to-all communication, where data is sent to all devices in a network.
3. How are multicast addresses assigned in routing?
Ans. Multicast addresses are assigned based on the Internet Group Management Protocol (IGMP) and the Multicast Listener Discovery (MLD) protocol. Devices interested in receiving multicast traffic join a multicast group by sending a membership report to their local router. The router then assigns a multicast address to the group and forward multicast traffic to all group members.
4. Can multicast addresses be used for routing within a local network only?
Ans. No, multicast addresses can be used for routing within both local networks and wide-area networks (WANs). Multicast routing protocols, such as Protocol Independent Multicast (PIM), enable the efficient distribution of multicast traffic across multiple networks, allowing devices in different locations to receive multicast data.
5. Are multicast addresses routable over the internet?
Ans. Yes, multicast addresses can be routable over the internet. However, due to the complexity and scalability challenges of multicast routing over the global internet, multicast traffic is typically limited to specific multicast-enabled networks or private networks. Internet Service Providers (ISPs) and network administrators need to implement specific multicast routing protocols and configurations to support multicast routing over the internet.
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