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Cisco Networking Essentials by Troy McMillan

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Exploring Application Layer Protocols

Protocols that operate at the Application layer are dedicated to a very specific function. For example, DNS, which gets its name from the hierarchical system that its uses called the Domain Name System, does nothing but handle requests from computers that are trying to match an IP address with a computer name (which is discussed in detail later in this section).

Application layer protocols have a dependent relationship with the protocols that operate at the lower levels of the TCP/IP model. There is no mechanism built into these protocols to locate the source and destination and get the application data where it needs to go. The Application layer protocol depends on TCP, UDP, IP, and ARP to take care of that ...

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