Application Layer Protocols
The application layer is the topmost layer of both the OSI and TCP/IP models. It provides services directly to user applications. Common protocols include HTTP, FTP, SMTP, DNS, DHCP, and SNMP.
DNS
The Domain Name System (DNS) translates domain names (e.g., www.example.com) to IP addresses. DNS uses a hierarchical namespace with root servers, TLD servers, and authoritative servers. Queries can be recursive or iterative. Resource records include A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), and NS (name server).
HTTP and HTTPS
HTTP (Hypertext Transfer Protocol) is the foundation of the World Wide Web. It is a request-response protocol using methods like GET, POST, PUT, DELETE. HTTP/1.1 supports persistent connections. HTTP/2 adds multiplexing and header compression. HTTPS adds TLS encryption for secure communication.
FTP
FTP (File Transfer Protocol) transfers files between client and server using two connections: a control connection (port 21) for commands and a data connection (port 20) for file transfer. FTP supports ASCII and binary modes. SFTP and FTPS add security.
Email Protocols
SMTP (Simple Mail Transfer Protocol, port 25) sends email from client to server and between servers. POP3 (Post Office Protocol, port 110) downloads mail to the client and optionally deletes from server. IMAP (Internet Message Access Protocol, port 143) keeps mail on server and supports folder management.
DHCP
DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses and network configuration to hosts. The process follows four steps: DISCOVER → OFFER → REQUEST → ACKNOWLEDGE (DORA). DHCP leases addresses for a configurable period.
SNMP and Telnet
SNMP (Simple Network Management Protocol) monitors and manages network devices. It uses agents, managers, and MIBs (Management Information Bases). Telnet (port 23) provides remote terminal access but sends data in plaintext. SSH (port 22) is its secure replacement.
Peer-to-Peer and Client-Server
The client-server model has a dedicated server providing services to clients (web, email, database). The peer-to-peer model has each node acting as both client and server (BitTorrent, blockchain). Hybrid models combine both approaches.
Summary
Application layer protocols define how applications communicate over the network. DNS resolves names, HTTP serves web content, SMTP/POP3/IMAP handle email, DHCP automates configuration, and SNMP manages devices.