Ccna routing and Switching Complete Study Guide

Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP) finds the hardware address of a host from a known IP 

address. Here’s how it works: When IP has a datagram to send, it must inform a Network 

Access protocol, such as Ethernet or wireless, of the destination’s hardware address on the 

local network. Remember that it has already been informed by upper-layer protocols of the 

destination’s IP address. If IP doesn’t find the destination host’s hardware address in the 

ARP cache, it uses ARP to find this information.

As IP’s detective, ARP interrogates the local network by sending out a broadcast asking 

the machine with the specified IP address to reply with its hardware address. So basically, 

ARP translates the software (IP) address into a hardware address—for example, the des-

tination machine’s Ethernet adapter address—and from it, deduces its whereabouts on the 

LAN by broadcasting for this address. Figure 3.19 shows how an ARP broadcast looks to a 

local network.

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TCP/IP and the DoD Model 

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f I g u r e   3 .19     Local ARP broadcast

I need the Ethernet

address of 10.1.1.2.

10.1.1.1

IP: 10.1.1.2 = ???

IP: 10.1.1.2

Ethernet: 45:AC:24:E3:60:A5

10.1.1.2

I heard that broadcast.

The message is for me.

Here is my Ethernet address.

ARP resolves IP addresses to Ethernet (MAC) addresses.

The following trace shows an ARP broadcast—notice that the destination hardware 

address is unknown and is all Fs in hex (all 1s in binary)—and is a hardware address 

broadcast:

 Flags:         0x00

 Status:        0x00

 Packet Length: 64

 Timestamp:     09:17:29.574000 12/06/03

Ethernet Header

 Destination:   FF:FF:FF:FF:FF:FF Ethernet Broadcast

 Source:        00:A0:24:48:60:A5

 Protocol Type: 0x0806 

IP ARP

ARP - Address Resolution Protocol

 Hardware:                1 Ethernet 

(10Mb)

 Protocol:                0x0800 

IP

 Hardware Address Length: 6

 Protocol Address Length: 4

 Operation:               1 

ARP Request

 Sender Hardware Address: 00:A0:24:48:60:A5

 Sender Internet Address: 172.16.10.3

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  Introduction to TCP/IP 

 Target Hardware Address: 00:00:00:00:00:00 

(ignored)

 Target Internet Address: 172.16.10.10

Extra bytes (Padding):

 ................ 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A

  0A 0A 0A 0A 0A

Frame Check Sequence: 0x00000000

IP Addressing

One of the most important topics in any discussion of TCP/IP is IP addressing. An IP 

address is a numeric identifier assigned to each machine on an IP network. It designates the 

specific location of a device on the network.

An IP address is a software address, not a hardware address—the latter is hard-coded on 

a network interface card (NIC) and used for finding hosts on a local network. IP addressing 

was designed to allow hosts on one network to communicate with a host on a different 

network regardless of the type of LANs the hosts are participating in.

Before we get into the more complicated aspects of IP addressing, you need to under-

stand some of the basics. First I’m going to explain some of the fundamentals of IP address-

ing and its terminology. Then you’ll learn about the hierarchical IP addressing scheme and 

private IP addresses.