4.1 Introduction

4.1 Introduction

The problem that we deal with in this chapter is that IP addresses only make sense to the TCP/IP protocol suite. A data link such as an Ethernet or a token ring has its own addressing scheme (often 48-bit addresses) to which any network layer using the data link must conform. A network such as an Ethernet can be used by different network layers at the same time. For example, a collection of hosts using TCP/IP and another collection of hosts using some PC network software can share the same physical cable.

When an Ethernet frame is sent from one host on a LAN to another, it is the 48-bit Ethernet address that determines for which interface the frame is destined. The device driver software never looks at the destination IP address in the IP datagram.

Address resolution provides a mapping between the two different forms of addresses: 32-bit IP addresses and whatever type of address the data link uses. RFC 826 [Plummer 1982] is the specification of ARP.

Figure 4.1 shows the two protocols we talk about in this chapter and the next : ARP (address resolution protocol) and RARP (reverse address resolution protocol).

Figure 4.1. Address resolution protocols: ARP and RARP.
graphics/04fig01.gif

ARP provides a dynamic mapping from an IP address to the corresponding hardware address. We use the term dynamic since it happens automatically and is normally not a concern of either the application user or the system administrator.

RARP is used by systems without a disk drive (normally diskless workstations or X terminals) but requires manual configuration by the system administrator. We describe it in Chapter 5.



TCP.IP Illustrated, Volume 1. The Protocols
TCP/IP Illustrated, Vol. 1: The Protocols (Addison-Wesley Professional Computing Series)
ISBN: 0201633469
EAN: 2147483647
Year: 1993
Pages: 378

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