TCP IP Illustrated

43 arp cache essential to the efficient operation of

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Unformatted text preview: net addresses to hardware addresses. The normal expiration time of an entry in the cache is 20 minutes from the time the entry was created. We can examine the ARP cache with the arp(8) command. The -a option displays all entries in the cache: bsdi % arp -a sun ( at 8:0:20:3:f6:42 svr4 ( at 0:0:c0:c2:9b:26 The 48-bit Ethernet addresses are displayed as six hexadecimal numbers separated by colons. We discuss additional features of the arp command in Section 4.8. 4.4 ARP Packet Format Figure 4.3 shows the format of an ARP request and an ARP reply packet, when used on an Ethernet to resolve an IP address. (ARP is general enough to be used on other networks and can resolve addresses other than IP addresses. The first four fields following the frame type field specify the types and sizes of the final four fields.) Figure 4.3 Format of ARP request or reply packet when used on an Ethernet. The first two fields in the Ethernet header are the source and destination Ethernet addresses. The special Ethernet destination address of all one bits means the broadcast address. All Ethernet interfaces on the cable receive these frames. The 2-byte Ethernet frame type specifies the type of data that follows. For an ARP request or an ARP reply, this field is 0x0806. The adjectives hardware and protocol are used to describe the fields in the ARP packets. For example, an ARP request asks for the protocol address (an IP address in this case) corresponding to a hardware address (an Ethernet address in this case). The hard type field specifies the type of hardware address. Its value is 1 for an Ethernet. Prot file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/arp_addr.htm (4 of 12) [12/09/2001 14.46.39] Chapter 4. ARP: Address Resolution Protocol type specifies the type of protocol address being mapped. Its value is 0x0800 for IP addresses. This is purposely the same value as the type field of an Ethernet frame containing an IP datagram. (See Figure 2.1.) The next...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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