Ccna routing and Switching Complete Study Guide

IP Type:             0x01 ICMP

 Header Checksum:     0x2df0

 Source IP Address:   100.100.100.2

 Dest. IP Address:    100.100.100.1

 No Internet Datagram Options

ICMP - Internet Control Messages Protocol

 ICMP Type:       8 Echo Request

 Code:            0

 Checksum:        0x395c

 Identifier:      0x0300

 Sequence Number: 4352

 ICMP Data Area:

 abcdefghijklmnop  61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70

 qrstuvwabcdefghi  71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69

Frame Check Sequence: 0x00000000

Notice anything unusual? Did you catch the fact that even though ICMP works at the 

Internet (Network) layer, it still uses IP to do the Ping request? The Type field in the IP 

header is 

0x01

, which specifies that the data we’re carrying is owned by the ICMP protocol. 

Remember, just as all roads lead to Rome, all segments or data must go through IP!

The Ping program uses the alphabet in the data portion of the packet as a 

payload, typically around 100 bytes by default, unless, of course, you are 

pinging from a Windows device, which thinks the alphabet stops at the let-

ter W (and doesn’t include X, Y, or Z) and then starts at A again. Go figure!

If you remember reading about the Data Link layer and the different frame types in 

Chapter 2, “Ethernet Networking and Data Encapsulation,” you should be able to look 

at the preceding trace and tell what type of Ethernet frame this is. The only fields are 

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Chapter 3 

■

  Introduction to TCP/IP 

destination hardware address, source hardware address, and Ether-Type. The only frame 

that uses an Ether-Type field exclusively is an Ethernet_II frame.

We’ll move on soon, but before we get into the ARP protocol, let’s take another look 

at ICMP in action. Figure 3.18 shows an internetwork—it has a router, so it’s an internet-

work, right?

f I g u r e   3 .18     ICMP in action

Server 1

10.1.2.2/24

10.1.4.2/24

I’m trying to

telnet to

10.1.1.5—

can you

forward this

request?

10.1.5.2/24

10.1.5.3/24

10.1.5.4/24

10.1.5.5/24

10.1.1.0 network?

No, never heard of it!

Discard! Create ICMP packet!

Server 1 (10.1.2.2) telnets to 10.1.1.5 from a DOS prompt. What do you think Server 1 

will receive as a response? Server 1 will send the Telnet data to the default gateway, which 

is the router, and the router will drop the packet because there isn’t a network 10.1.1.0 in 

the routing table. Because of this, Server 1 will receive an ICMP destination unreachable 

back from the router.