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cisco ccna vs comptia a+.ICMPEcho Request message IP --.Figure 1-62 Two Hosts Exchanging ICMP EchoRequest and Echo Reply > Shaw InformationICMP also has a variety of common DestinationUnreachable messages. Such ICMP messages are encountered in many situations and are further classified as ICMP Destination Host Unreachable messages, ICMP Destination Network Unreachable messages, ICMP Destination Port Unreachable messages, and so on. For example, if a remote host is pinged on host A, if host A's default gateway does not have a route to the remote destination host, it replies to host A with an ICMP DestinationUnreachable message, a kind of ICMP Destination NetworkUnreachable message.1.5 Internet Control Message Protocol (ICMP) 73message (type 3, code 0), indicating that the ping packet (ICMP74 Chapter 1 Network Protocols and Network Device BasicsEcho Request message) to the destination remote host and has been dropped. When an IP packet with an "expired" TTL field value is received, the router generates an ICMP Time Exceeded message. The router discards the "expired" packet and sends back an Exceed inTransit message (type 11, code 0), which is a type of ICMP expiration message, to its source IP address.1.6 Domain Name System (DNS)In all of the previous examples, whenever communication between IP hosts is involved, the IP address of the destination host is always specified directly. However, it is not always easy to remember individual IP addresses. Imagine that the reader is browsing the official website of the Asynchronous community, and 123.57.69.10 and www.epubit.com?There is a solution called the Domain Name System (DNS). DNS is a hierarchical, distributed database whose purpose is to establish correspondence between IP addresses and the names of devices that have that IP address. This section describes DNS and its basic functions. DNS operates at the application layer of the TCP/IP model, and this section contains DNS