NET 125 Module 13: ICMP

Internet Control Message Protocol (ICMP)

provides feedback about issues related to the
processing of IP packets under certain conditions

ICMPv4 is the messaging protocol for

IPv4

ICMPv6 is the messaging protocol for

IPv6

The ICMP messages common to both ICMPv4 and ICMPv6 include:

• Host reachability
• Destination or Service Unreachable
• Time exceeded

T or F: Note: ICMPv4 messages are not required and are often not allowed within a network for
security reasons

True

ICMP Echo Message

used to test the reachability of a host on an IP
network

In the example:
• The local host sends an ICMP Echo
Request to a host.
• If the host is available, the
destination host

responds with an Echo Reply

An ICMP Destination Unreachable message can be used to notify the source that

a destination or service is unreachable

The ICMP message will include a code indicating why the packet could not be

delivered

A few Destination Unreachable codes for ICMPv4 are as follows:

• 0 -

• 1 -

• 2 -

• 3 -

• 0 = Net unreachable

• 1 = Host unreachable

• 2 = Protocol unreachable

• 3 = Port unreachable

When the Time to Live (TTL) field in a packet is decremented to 0, an

ICMPv4 Time Exceeded message will be sent to the source host

Messaging between an IPv6 router and an
IPv6 device, including dynamic address
allocation are as follows:

• Router Solicitation (RS) message
• Router Advertisement (RA) message

Messaging between IPv6 devices, including
duplicate address detection and address
resolution are as follows:

• Neighbor Solicitation (NS) message

• Neighbor Advertisement (NA) message

RA messages are sent by IPv6-enabled
routers every 200 seconds to provide
addressing information to

IPv6-enabled hosts

RA message can include addressing
information for the host such as the

prefix, prefix length, DNS address, and domain
name.

A host using Stateless Address
Autoconfiguration (SLAAC) will set its
default gateway to the

link-local address of
the router that sent the RA

An IPv6-enabled router will also send out
an RA message in response to an

RS message

A device assigned a global IPv6 unicast or
link-local unicast address, may perform
duplicate address detection (DAD) to

ensure that the IPv6 address is unique

To check the uniqueness of an address, the
device will send an NS message with its
own IPv6 address

as the targeted IPv6 address

If another device on the network has this
address, it will respond with an NA
message notifying to the sending device
that

the address is in use

to determine the MAC address for the
destination, the device will send an NS
message to the

solicited node address

ping command

IPv4 and IPv6 testing utility that uses ICMP echo request and echo reply messages to test connectivity between
hosts and provides a summary that includes the
success rate and average round-trip time to the
destination

A successful ping to the default gateway
indicates

that the host and the router
interface serving as the default gateway
are both operational on the local network

If the default gateway address does not
respond

a ping can be sent to the IP
address of another host on the local
network that is known to be operational

Traceroute

a utility that is used
to test the path between two hosts and
provide a list of hops that were
successfully reached along that path

The ICMP messages common to both ICMPv4 and ICMPv6 are:

Host reachability,
Destination or Service Unreachable, and Time exceeded

Ping (used by IPv4 and IPv6) uses ICMP echo request and echo reply messages to

test
connectivity between hosts