- Introduction
- The Basics of a Network
- Basic Network Utilities
- The OSI Model
- What Does This Mean for Security?
- Assessing Likely Threats to the Network
- Classifications of Threats
- Likely Attacks
- Threat Assessment
- Understanding Security Terminology
- Choosing a Network Security Approach
- Network Security and the Law
- Using Security Resources
- Summary
This chapter is from the book
This chapter is from the book
This chapter is from the book
The OSI Model
The Open Systems Interconnect (OSI) model describes how networks communicate (see Table 1-3). It describes the various protocols and activities and tells how the protocols and activities relate to each other. This model is divided into seven layers. It was originally developed by the International Organization for Standardization (ISO) in the 1980s.
TABLE 1-3 The OSI Model
Layer |
Description |
Protocols |
Application |
This layer interfaces directly to applications and performs common application services for the application processes. |
POP, SMTP, DNS, FTP, Telnet |
Presentation |
The presentation layer relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. |
Telnet, Network Data Representation (NDR), Lightweight Presentation Protocol (LPP) |
Session |
The session layer provides the mechanism for managing the dialogue between end-user application processes. |
NetBIOS |
Transport |
This layer provides end-to-end communication control. |
TCP, UDP |
Network |
This layer routes the information in the network. |
IP, ARP, ICMP |
Data link |
This layer describes the logical organization of data bits transmitted on a particular medium. The data link layer is divided into two sublayers: the Media Access Control layer (MAC) and the Logical Link Control layer (LLC). |
SLIP, PPP |
Physical |
This layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. In other words, the physical layer is the actual NIC, Ethernet cable, and so forth. |
IEEE 1394, DSL, ISDN |
Many networking students memorize this model. At least memorizing the names of the seven layers and understanding basically what they each do is good. From a security perspective, the more you understand about network communications, the more sophisticated your defense can be. The most important thing for you to understand is that this model describes a hierarchy of communication. One layer communicates only with the layer directly above it or below it.
