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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Network Interface / Internet "Layer Connection" Protocols
           9  Address Resolution and the TCP/IP Address Resolution Protocol (ARP)
                9  Address Resolution Concepts and Issues

Previous Topic/Section
The Need For Address Resolution
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23
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Dynamic Address Resolution
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Address Resolution Through Direct Mapping
(Page 1 of 3)

Network layer addresses must be resolved into data link layer addresses numerous times during the travel of each datagram across an internetwork. We therefore want the process to be as simple and efficient as possible. The easiest method of accomplishing this is to do direct mapping between the two types of addresses.

The basic idea behind direct mapping is to choose a scheme for layer two and layer three addresses so that you can determine one from the other using a simple algorithm. This enables you to take the layer three address, and follow a short procedure to convert it into a layer two address. In essence, whenever you have the layer three address you already have the layer two address.

The simplest example of direct mapping would be if we used the same structure and semantics for both data link and network layer addresses. This is generally impractical, because the two types of addresses serve different purposes, and are therefore based on incompatible standards. However, we can still perform direct mapping if we have the flexibility of creating layer three addresses that are large enough to encode a complete data link layer address within them. Then, determining the layer two address is a simple matter of selecting a certain portion of the layer three address.

As an example, consider a simple LAN technology like ARCNet. It uses a short 8-bit data link layer address, with valid values of 1 to 255, which can be assigned by an administrator. We could easily set up an IP network on such a LAN by taking a class C (or /24) network and using the ARCNet data link layer as the last octet. So, if our network were, for example, 222.101.33.0/24, we could assign the device with ARCNet address #1 the IP address 222.101.33.1, the device with ARCNet address #29 would be 222.101.33.29 and so forth, as shown in Figure 45.


Figure 45: Address Resolution Through Direct Mapping

When the hardware address is small, it is easy to define a mapping that directly corresponds to a portion of a layer three address. In this example, an 8-bit MAC address, such as that used for ARCNet, is mapped to the last byte of the device’s IP address, making address resolution a trivial matter.

 


The appeal of this system is obvious. Conceptually, it is trivially simple to understand—to get the hardware address for a device, you just use the final eight bits of the IP address. It's also very simple to program devices to perform, and highly efficient, requiring no exchange of data on the network at all.

Key Concept: When the layer two address is smaller than the layer three address, it is possible to define a direct mapping between them, so that the hardware address can be determined directly from the network layer address. This makes address resolution extremely simple, but reduces flexibility in how addresses are assigned.



Previous Topic/Section
The Need For Address Resolution
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Dynamic Address Resolution
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