Lesson 4:AppleTalk

Like NetBEUI, AppleTalk is a protocol stack that was designed to provide small groups of computers with basic networking capabilities. Apple Macintosh systems have had integrated networking hardware and software almost since their introduction and, although AppleTalk does not have the flexibility of TCP/IP, it is simple to set up and use and provides adequate performance for standard networking tasks, such as file and printer sharing. AppleTalk does not support Internet communications, however, which is the primary reason it is currently being phased out in favor of TCP/IP.

After this lesson, you will be able to

  • Understand AppleTalk addressing
  • Diagram the Datagram Delivery Protocol header

Estimated lesson time: 10 minutes

AppleTalk originally used its own data-link layer protocol, called Apple LocalTalk, the adapter for which was built into the Macintosh computer. LocalTalk runs at only 230 Kbps, however, and it has come to be replaced by Apple EtherTalk at 10 Mbps (or Fast EtherTalk at 100 Mbps) and, to a lesser extent, TokenTalk at 4 or 16 Mbps and FDDITalk at 100 Mbps, which are adaptations of the Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) protocols.

Like IP and IPX, AppleTalk uses a hierarchical addressing system to identify the computers on a network. Every AppleTalk computer has a unique 8-bit node ID that it randomly selects and assigns to itself as it connects to the network. After transmitting a broadcast message to make sure that no other computer is using the same ID, the system stores the address for future use each time it reconnects. Because the number is only 8 bits long, a single AppleTalk network can have no more than 254 nodes (28 – 2, because 0 and 255 are not used for node IDs). AppleTalk also uses 16-bit network numbers to identify the LANs in an internetwork for routing purposes. A computer connecting to the network obtains the network number value for the LAN using the Zone Information Protocol (ZIP). As with IP, AppleTalk networks can be connected together with routers that read the destination network numbers and node IDs in each packet and forward them to the appropriate LAN.

To identify specific processes running on a computer, AppleTalk uses an 8-bit socket number, which performs the same function as the Protocol field in the IP header. The combination of network number, node ID, and socket is expressed as three decimal numbers separated by periods, as in 2.12.50, meaning network 2, node 12, and socket 50. AppleTalk reconciles the data-link hardware addresses coded into network interface adapters with the node IDs and network numbers using the AppleTalk Address Resolution Protocol (AARP), which functions remarkably like the TCP/IP Address Resolution Protocol (ARP).

In addition to the node IDs and network numbers, AppleTalk computers have friendly names that make it easier to locate specific resources on the network. Computers have their own names and groups of computers are gathered into units called zones. A zone is a logical grouping that makes it easier to locate specific resources on the network.

At the network layer, AppleTalk uses the DDP. Like IP and IPX, DDP is a connectionless protocol that encapsulates data generated by an upper layer protocol and provides many of the same services as IP and IPX, including packet addressing, routing, and protocol identification. A simple AppleTalk network that consists of only one network number and one zone is called a nonextended network. A network that consists of multiple network numbers and zones is called an extended network, and uses the long-format DDP header shown in Figure 6.7.

Figure 6.7  The DDP long-format header

The functions of the DDP header fields are as follows:

  • Hop Count (1 byte).  Specifies the number of routers that have processed the packet on the way to its destination
  • Datagram Length (2 bytes).  Specifies the length of the DDP datagram; used for basic error detection
  • Checksum (2 bytes).  Optional field containing a checksum computed on the entire datagram; used for more extensive error detection
  • Source Socket Number (1 byte).  Specifies the socket number of the application or process that generated the information in the data field
  • Destination Socket Number (1 byte).  Specifies the socket number of the application or process to which the information in the data field is to be delivered
  • Source Address (3 bytes).  Specifies the network number and node ID of the computer generating the packet
  • Destination Address (3 bytes).  Specifies the network number and node ID of the computer that is to receive the packet
  • DDP Type (1 byte).  Identifies the upper layer protocol that generated the information carried in the data field
  • Data (variable, up to 586 bytes).  Contains information generated by an upper layer protocol

On a nonextended network, DDP uses the short-format header, which includes only the four source and destination fields, plus the datagram length and DDP type.

Exercise 1: Choosing a Network Layer Protocol

Specify which network layer protocol you would use on each of the following networks, and explain why.

  1. A private internetwork with mixed Windows and NetWare systems
  2. A two-node home Windows-based network with individual dial-up Internet connections
  3. A 10-node Windows-based network with a router connecting it to the Internet
  4. A five-node Macintosh network that is not connected to the Internet

Lesson Review

  1. Which of the following network layer protocols is not routable?
    1. IP
    2. IPX
    3. NetBEUI
    4. DDP
  2. Which of the following DDP fields identifies the upper layer protocol that generated the information in the data field?
    1. Source Socket Number
    2. Checksum
    3. Source Address
    4. DDP Type
  3. A LocalTalk network transmits data at what speed?
    1. 56 Kbps
    2. 230 Kbps
    3. 10 Mbps
    4. 16 Mbps

Lesson Summary

  • AppleTalk networks can use any one of several protocols at the data-link layer, including LocalTalk, EtherTalk, TokenTalk, and FDDITalk.
  • Computers on an AppleTalk network have 8-bit node IDs that they assign to themselves, and the networks have 16-bit network numbers.
  • Processes on AppleTalk computers are identified using 8-bit socket numbers.

Network+ Certification Training Kit
Self-Paced Training Kit Exam 70-642: Configuring Windows Server 2008 Network Infrastructure
ISBN: 0735651604
EAN: 2147483647
Year: 2001
Pages: 105

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