Another Layer of Addressing

Another Layer of Addressing

Data Link Layer
OSI Data Link layer (Layer 2) physical addressing, implemented as an Ethernet MAC address, is used to transport the frame across the local media. Although providing unique host addresses, physical addresses are non-hierarchical. They are associated with a particular device regardless of its location or to which network it is connected.

These Layer 2 addresses have no meaning outside the local network media. A packet may have to traverse a number of different Data Link technologies in local and wide area networks before it reaches its destination. A source device therefore has no knowledge of the technology used in intermediate and destination networks or of their Layer 2 addressing and frame structures.

Network Layer
Network layer (Layer 3) addresses, such as IPv4 addresses, provide the ubiquitous, logical addressing that is understood at both source and destination. To arrive at its eventual destination, a packet carries the destination Layer 3 address from its source. However, as it is framed by the different Data Link layer protocols along the way, the Layer 2 address it receives each time applies only to that local portion of the journey and its media.

In short:
The Network layer address enables the packet to be forwarded toward its destination.
The Data Link layer address enables the packet to be carried by the local media across each segment.

In Ethernet, different MAC addresses are used for Layer 2 unicast, multicast, and broadcast communications.

Unicast
A unicast MAC address is the unique address used when a frame is sent from a single transmitting device to single destination device.

In the example shown in the figure, a host with IP address 192.168.1.5 (source) requests a web page from the server at IP addresses 192.168.1.200. For a unicast packet to be sent and received, a destination IP address must be in the IP packet header. A corresponding destination MAC address must also be present in the Ethernet frame header. The IP address and MAC address combine to deliver data to one specific destination host.

Broadcast
With a broadcast, the packet contains a destination IP address that has all ones (1s) in the host portion. This numbering in the address means that all hosts on that local network (broadcast domain) will receive and process the packet. Many network protocols, such as Dynamic Host Configuration Protocol (DHCP) and Address Resolution Protocol (ARP), use broadcasts. How ARP uses broadcasts to map Layer 2 to Layer 3 addresses is discussed later in this chapter.

As shown in the figure, a broadcast IP address for a network needs a corresponding broadcast MAC address in the Ethernet frame. On Ethernet networks, the broadcast MAC address is 48 ones displayed as Hexadecimal FF-FF-FF-FF-FF-FF.

Multicast
Recall that multicast addresses allow a source device to send a packet to a group of devices. Devices that belong to a multicast group are assigned a multicast group IP address. The range of multicast addresses is from 224.0.0.0 to 239.255.255.255. Because multicast addresses represent a group of addresses (sometimes called a host group), they can only be used as the destination of a packet. The source will always have a unicast address.

Examples of where multicast addresses would be used are in remote gaming, where many players are connected remotely but playing the same game, and distance learning through video conferencing, where many students are connected to the same class.

Another Layer of Addressing
Another Layer of Addressing
Another Layer of Addressing
Another Layer of Addressing
Another Layer of Addressing
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