Physical Addresses

A physical address is a unique identifier hard-coded into the NIC of each node. Its other common names are:

The designers of the most popular MAC-layer protocols (Ethernet, Token Ring, and Fiber Distributed Data Interface [FDDI]) have allocated 48 bits for the hardware address. Each NIC comes with a hardware address preconfigured from the factory. NIC manufacturers register hardware addresses with a worldwide central authority to guarantee the numbers they assign do not conflict with those of any other manufacturer. This guarantees each hardware address is globally unique.

It would be natural to want to associate the term "physical address" with the Physical Layer. However, the Physical Layer is only concerned with transmitting and receiving bits from the physical medium, and does not "see" bits as organized into meaningful patterns, such as an address. The physical address, or hardware address, is actually processed by OSI Layer 2, the Data Link Layer. We review the OSI model in the next lesson. This hardware address is the address ultimately required for frames to be delivered to a destination network node.

   Logical Addresses

Logical addresses are symbolic identifiers. These are assigned by software and are used by processes operating at OSI Layer 3 and above. There are two primary types of logical addresses:

Data often starts out (at the higher layers) addressed to some symbolic name, such as the host name in the command Telnet Serverhost. The name "serverhost" is the logical address of the destination the user is attempting to contact using the telnet (TCP/IP) application and protocol. But if the message is actually to be delivered to this host, the sending computer must somehow discover the destination's physical address. In this case, an intermediate logical address (the IP address) will first be derived from the symbolic name using some sort of a name service process, such as Domain Name System (DNS). Then a protocol such as Address Resolution Protocol (ARP) can find the hardware address that corresponds to that IP address. When the sending node knows all of these addresses, it can finally transmit the data to its destination.

The most important fact to remember about logical addresses is that a logical address will not get the information "into the box." Only a hardware address used by the Data Link Layer, whether a broadcast address, multicast (group) address, or unicast (individual) address, can physically deliver a frame to the destination device.

   Layers of Addresses

Physical and logical addresses work together to transmit information from source to destination within a segment of a network. As an example, consider how a Web server returns data in response to a client request. The server responds by sending a frame of information across an Ethernet network to the client that made the request. This response is shown on the Web Page Response Diagram.

This diagram demonstrates the correlation between clients, client applications and client processes, client protocols and corresponding servers and server applications, server processes, and server protocols. On the server side of the diagram, the computer is running some type of Web server software such as Apache or Internet Information Server (IIS). The software consists of not only the application, but the protocols needed to send Web documents to the client. The application interfaces with HTTP, which is responsible for responding to the client with the appropriate information.

The HTTP process running on the server creates an HTTP header that contains protocol information used to communicate with the peer HTTP process running on the client. HTTP on the server uses TCP to establish a connection with the client, and reliably transfer the HTTP response to the client software. Thus, the TCP process running on the Web server communicates with the TCP process running on the client.

TCP on the server communicates with IP on the server to transmit the TCP message across the network, packet by packet. IP on the Web server indirectly communicates with its IP peer on the client. IP on the Web server also communicates with the Web server's Ethernet driver. The Ethernet driver is responsible for transmitting a frame of information, consisting of the packet and message, to the next node in the network across a physical link. It does this by relying on the services of the Ethernet NIC and the Physical Layer (the cables and connectors). On the Web Page Response Diagram, both the Web server and client NICs are located on the same physical segment.

In this scenario, three addresses are used by the sending and receiving computers to communicate between application processes. At the lowest level, the Ethernet processes, located on the NICs of the server and client, use Ethernet physical addresses to transmit a frame from NIC to NIC. Each frame contains an IP packet, or portion of a packet.

IP addresses indicate which host on the network should get each packet located inside the Ethernet frame. Each packet contains a TCP message, or portion of a message. As the receiving IP process receives all packets that make up the TCP message, it passes the messages or fragments up to TCP.

TCP reassembles the original message, then passes it to the destination software process address (port). In this case, the data is addressed to the HTTP process at Port 80.

What do the destination and source addresses of an Ethernet frame indicate?

   Activities

See the Activities and Extended Activities section in Unit 1 Lesson 2 in your textbook Local Area Networks to test what you have learned so far.

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