Application Layer

Application Layer: It is the topmost layer of the OSI model and it directly interacts with web applications or application services. This layer provides several ways for manipulating the data (information) which actually enables any type of user to access the network with ease.

Functions of Application Layer: 

• Application Layer facilitates email sending and storing features. 
• This layer allows users to access, retrieve and manage files from a remote computer.
• This layer provides services that include: E-mail, File transfer, directory services, network resources, etc. 

Domain Name Server: DNS is a hostname for IP address translation service. DNS is a distributed database implemented in a hierarchy of name servers. It is an application layer protocol for message exchange between clients and servers.

Protocols in Application Layer

• Dynamic Host Configuration Protocol(DHCP) is an application layer protocol that is used to provide:
  • Subnet Mask (Option 1 – e.g., 255.255.255.0)
  • Router Address (Option 3 – e.g., 192.168.1.1)
  • DNS Address (Option 6 – e.g., 8.8.8.8)
  • Vendor Class Identifier (Option 43 – e.g., ‘unifi’ = 192.168.1.9 ##where unifi = controller)
• Simple Network Management Protocol (SNMP): SNMP is an application layer protocol that uses UDP port number 161/162.SNMP is used to monitor networks, detect network faults, and sometimes even used to configure remote devices.
• Simple Mail Transfer Protocol (SMTP): SMTP is an application layer protocol. The client who wants to send the mail opens a TCP connection to the SMTP server and then sends the mail across the connection. The SMTP server is always in listening mode. As soon as it listens for a TCP connection from any client, the SMTP process initiates a connection on that port (25). After successfully establishing the TCP connection the client process sends the mail instantly.
• File Transfer Protocol (FTP): File Transfer Protocol(FTP) is an application layer protocol that moves files between local and remote file systems. It runs on top of TCP, like HTTP. To transfer a file, 2 TCP connections are used by FTP in parallel: control connection and data connection.
• Hypertext Transfer Protocol (HTTP): is an application-level protocol that uses TCP as an underlying transport and typically runs on port 80. HTTP is a stateless protocol i.e. server maintains no information about past client requests.

Port number and transport mode used by Protocols:

See Last Minute Notes on all subjects here. 

1. OSI Model
2. Physical Layer
3. Data Link layer
4. Network Layer
5. Transport Layer
6. Presentation & Session layer
7. Application Layer