The application layer is the top layer of the OSI model. It provides network services that software and users interact with directly, including web browsing, email, file transfer, and name resolution.
Common Application-Layer Protocols
- HTTP/HTTPS: web communication.
- DNS: converts names into IP addresses.
- SMTP, IMAP, and POP: email delivery and retrieval.
- FTP/SFTP: file transfer.
- DHCP: automatic IP configuration.
Why It Matters
When an application fails, the network path may still be healthy. Check the service, DNS resolution, authentication, ports, and firewall rules before assuming that the physical network is the cause.