Page 159 - Computer_Science_F5

Computer Science Address Resolution Protocol (ARP) (a) ICMP echo request (Ping)
Sending a message to a friend at a
Purpose: To check if a destination
party requires knowing their location.
device is reachable.
Similarly, ARP translates IP addresses
Process: A source device sends an ICMP
to MAC addresses for direct network
FOR ONLINE READING ONLY
communication, as shown in Figure 3.13.
device. If the target device is reachable
For example: Echo Request message to the target
(a) Device A with IP address 2.2.2.2 and operational, it responds with an
wants to send data to device B with ICMP Echo Reply message.
IP address 4.4.4.4.
(b) Device A sends an ARP request for (b) ICMP echo reply (Pong)
device B’s MAC address. Purpose: To confirm that the target
(c) Device B responds with its MAC device is reachable and responsive.
Process: Upon receiving an ICMP
address. Echo Request, the target device sends
(d) Device A sends data directly to device an ICMP echo reply back to the source
B. device, as shown in Figure 3.14.

IP 1.1.1.1 IP 3.3.3.3 ICMP echo-request
MAC a:a:a:a:a:a:a MAC b:b:b:b:b:b:b

Host Router Server
Network ICMP echo-reply
segment
ARP request Figure 3.14: ICMP implementation
for 4.4.4.4
ARP reply
d:d:d:d:d:d:d (c) Destination unreachable
Purpose: To notify the source device
IP 2.2.2.2 IP 4.4.4.4 that the destination cannot be reached.
MAC c:c:c:c:c:c:c MAC d:d:d:d:d:d:d

Figure 3.13: Network segment Process: If a router or gateway cannot
deliver a packet to the destination, it
Internet Control Message Protocol sends an ICMP Destination Unreachable
(ICMP) message back to the source device.

ICMP monitors network issues and
reports them back. It uses IP packets for (d) Time exceeded
transportation and includes tools like: Purpose: To indicate that a packet has
been discarded because it has exceeded
ICMP Mechanisms the maximum time to live (TTL) value.
ICMP messages are encapsulated within
IP packets. Some of the most common Process: If a packet’s TTL value reaches
ICMP, include: zero before it reaches its destination, the

150
for Advanced Secondary Schools

Computer Science Form 5.indd 150 23/07/2024 12:33