Welcome to TCP/IP Part 5

Internet Control Message Protocol (ICMP) works at the Network Layer (Layer 3) and is used by Internet Protocol for several different purposes. ICMP is a managing protocol and messaging service provider for Internet Protocol.  The ICMP messages are carried as IP datagrams that afford a host’s capability to discover routes to gateways.  ICMP packets can provide hosts with information about network problems and are encapsulated within IP datagrams.

Destination Unreachable is where a router cannot send an IP datagram any further to its intended destination, it therefore uses ICMP to send a message back to the sender advising it that the destination Host is unreachable.

When Host A sends a packet whose destination is Host B, the Lab_B router is what sends and ICMP destination unreachable message back to the sending device, or Host A.


Buffer Full is the message sent out to the sending Host by using ICMP and will continue to do so until the congestion has subsided.


Hops is the number of routers and IP datagram is permitted to travel, or pass through, if it reaches its limit before arriving at its destination Host the last router to receive that datagram then deletes or drops it.  That router will then use ICMP to send a message back to the sending Host of the loss of the datagram due to the maximum number of hops.


Ping (Packet Internet Groper) uses ICMP echo requests and reply messages to check both the physical and logical connectivity of a Host to a network, or internetwork. 


Traceroute uses ICMP time-outs and is used to discover the path a packet as it travels through an internetwork.


Perhaps it would be good to see the routing of a packet, please click on the URL below the ICMP packet figure.



ICMP in Action shows how the dropped packet will be handled.  Server 1 ( Telnets to a Host ( using the DOS prompt.  The packet will be sent to the default gateway, since the Server (1) has no knowledge as to where is located.  The default gateway will drop the packet because there is no listing of in the routing table of the router.  After dropping the packet, the router will send an ICMP packet to Server 1 stating that the destination is unreachable.

See also: Part 1, Part 2, Part 3, Part 4, Part 5


Works Cited

Lammle, T. (2007). CCNA Cisco Certified Network Associate Study Guide. Indianapolis: Wiley Publishing, Inc.

Odom, W. (2012). Official Cert Guide ICND1 640-822. Indianapolis, IN: Cisco Press.

Odom, W. (2011). Official Cert Guide ICND2 640-816. Indianapolis, IN: Cisco Press.



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