OSI Model : DNS Services and Protocol-III

How applications provide an interface for the user and provide access to the network, we will take a look at some specific commonly used protocols.

A port number is an addressing system used by the transport layer. Port numbers indicate apps and Application layer services that are the source and destination of data. Server applications often employ specified port numbers that are well-known to clients.

As we investigate the various TCP/IP Application layer protocols and services, we will refer to the TCP and UDP port numbers that are generally associated with these services.

⚡ Services in Application Layer📜

Domain Name System (DNS) - TCP/UDP Port 53
Hypertext Transfer Protocol (HTTP) - TCP Port 80
Simple Mail Transfer Protocol (SMTP) - TCP Port 25
Post Office Protocol (POP) - UDP Port 110
Telnet - TCP Port 23
Dynamic Host Configuration Protocol - UDP Port 67
File Transfer Protocol (FTP) - TCP Ports 20 and 21

⚡ DNS- Domain Name System 📜

Devices in data networks are assigned numeric IP addresses so that they may send and receive messages over the network. Most individuals, however, have difficulty memorizing this numerical address. As a result, domain names were developed in order to translate the numeric address into a simple, recognizable name.

OSI Model : DNS Services and Protocol-III

Fig 1.1- DNS Resolution

These domain names, such as www.cisco.com, are far easier for users to remember on the Internet than the actual numeric address for this server, 198.132.219.25. Furthermore, if Cisco decides to modify the numeric address, the user will be unaffected because the domain name would remain www.cisco.com.

The new address will simply be connected to the current domain name, preserving connection. Maintaining the mapping between domain names and the addresses they represented was an easy operation when networks were tiny. This manual technique, however, proved untenable as networks grew and the number of devices rose.

For these networks, the Domain Name System (DNS) was designed to resolve domain names to addresses. To resolve the names linked with these numerical addresses, DNS employs a dispersed network of servers.

The DNS protocol specifies an automated service for matching resource names with the necessary numeric network address. It specifies the format for requests, answers, and data. DNS protocol communications are carried out in a single format known as a message. This message format is used for all forms of client queries and server answers, error messages, and resource record information transmission between servers.

DNS is a client/server service; however, it differs from the other client/server services that we are examining. While other services use a client that is an application (such as web browser, e-mail client), the DNS client runs as a service itself. The DNS client, sometimes called the DNS resolver, supports name resolution for our other network applications and other services that need it.

When configuring a network device, we generally provide one or more DNS Server addresses that the DNS client can use for name resolution. Usually the Internet service provider provides the addresses to use for the DNS servers. When a user's application requests to connect to a remote device by name, the requesting DNS client queries one of these name servers to resolve the name to a numeric address.

The nslookup application in computer operating systems allows the user to manually query the name servers to resolve a specified host name. This tool may also be used to troubleshoot name resolution difficulties and to check the condition of the name servers.

When you use nslookup, the default DNS server for your host is shown. Then we may input the name of the host or domain for which we want the address. A query for www.cisco.com is made in the first query in the figure. The address is provided by the answering name server.

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OSI Model : DNS Services and Protocol-III