There are many more than three protocols which can operate at the application layer. Some of these are;
Telnet
FTP
TFTP
SMTP
IMAP
POP
DNS
RARP
BOOTP
SNMP
CMOT
Application Presentation Session
Layer 5 - Session layer
application
The three layers that I could think of are User interface, Client side, and server side.
FTP operates in application layer of ISO OSI layered model.
HTML is read by a browser and a browser works at the Application Layer.
DNS is an application layer protocol. The application layer of the OSI model is the layer closest to the user and provides network services to the applications of the user. Examples of protocols that operate at the application layer are: DNS, FTP, TFTP, SNMP, TELNET, RLOGIN, SMTP, MIME, NFS, FINGER, BOOTP and SMB.
The mail protocol, right? Yes, that is considered part of the application layer.
No, At each layer, protocols perform servers unique to that layer. While performing those services, the protocols also interact with protocols in the layers directly above and below. In addition, at the top of the OSI model, Application layer protocols interact with the software you use. At the bottom, Phsyical layer services act on the networking cables and connectors to issue and receive signals.
Telnet is an application layer protocol (Layer 7). Its functionality is similar to any of the layer 7 protocols, including DHCP, DNS, FTP, etc.
E-mail and FTP, like all client applications, operate at the Application Layer.
No. Only protocols that users can see on their screen are considered Presentation Layer based. Hubs and repeaters operate on the physical layer, because it doesn't do anything with the data, it simply forwards it.