Understanding Network Layers and Protocols
Network Layers Explained
Application Layer
- Defines the interfaces between the software application and communication functions of the network.
- Provides standardized services such as file transfer between systems.
Session Layer
- Manages user sessions and dialogues.
- Manages links between applications.
Network Layer
- Provides logical network addressing.
- Routes packets between networks by following a logical addressing scheme.
Physical Layer
- Defines the physical means to send data over network devices.
- Acts as an interface between devices and the network medium.
- Defines the mechanical, electrical, and optical characteristics for wired or wireless transmission.
Presentation Layer
- Standardizes data formats for use between different types of systems.
- Encodes and decodes user data, encrypts and decrypts data, compresses and decompresses data.
Transport Layer
- Manages the delivery of messages from end to end across the network.
- Provides reliable and sequenced delivery of packets through flow control and error recovery.
Data Link Layer
- Establishes procedures for communication links.
- Detects and corrects errors in the transmission of frames.
Key Network Protocols
Application Protocol: HTTP
The Hypertext Transfer Protocol (HTTP) governs the way web clients and servers interact. HTTP defines the format of requests and responses exchanged between the client and the server. HTTP relies on other protocols to regulate message transmission.
Transport Protocol: TCP
The Transmission Control Protocol (TCP) manages individual conversations between web servers and web clients. TCP segments HTTP formatted messages for transmission.
Internetwork Protocol: IP
The most common internetwork protocol is the Internet Protocol (IP). IP is responsible for formatting TCP segments, assigning logical addresses, and encapsulating them in packets for routing to the destination host.
Protocols for Network Access: Ethernet
Ethernet is the most widely used protocol for local networks. Network access protocols perform two major functions: data link management and physical network transmission.
Simple Mail Transfer Protocol (SMTP)
SMTP is used by an email client to send messages to a local mail server. The local server then determines if the message is directed to a local mailbox or a mailbox on another server.
If the server must send the message to a different server, SMTP is also used between the two servers. Applications send messages to the SMTP port 25.
Post Office Protocol (POP3)
A client that supports POP3 receives and stores messages for its users. When the client connects to the email server, messages are downloaded to the client. By default, messages are not retained on the server.
Internet Message Access Protocol (IMAP4)
A server that supports IMAP also receives and stores messages intended for its users. However, messages are stored in mailboxes on the server unless deleted. The latest version is IMAP4, which expects client requests on port 143.