Đang chuẩn bị liên kết để tải về tài liệu:
CompTIA Network+ Certification Study Guide part 42

CompTIA’s Network+ certification Study Guide part 42 is a globally-recognized, vendor neutral exam that has helped over 235,000 IT professionals reach further and higher in their careers. The 2009 Network+ exam (N10-004) is a major update with more focus on security and wireless aspects of networking. Our new study guide has been updated accordingly with focus on network, systems, and WAN security and complete coverage of today’s wireless networking standards. | 396 CHAPTER 9 Security Standards and Services DAMAGE AND DEFENSE. Denial of Service Attacks A port is a connection point into a device. Ports can be physical such as serial ports or parallel ports or they can be logical. Logical ports are ports used by networking protocols to define a network connection point to a device. Using Transmission Control Protocol Internet Protocol TCP IP both TCP and User Datagram Protocol UDP logical ports are used as connection points to a network device. Because a network device can have thousands of connections active at any given time these ports are used to differentiate between the connections to the device. A port is described as well known for a particular service when it is normal and common to find that particular software running at that particular port number. For example Web servers run on port 80 by default and File Transfer Protocol FTP file transfers use ports 20 and 21 on the server when it is in active mode. In passive mode the server uses a random port for data connection and port 21 for the control connection. Exam Warning There are two modes in which FTP operates active and passive. Active Mode 1. The FTP client initializes a control connection from a random port higher than 1024 to the server s port 21. 2. The FTP client sends a PORT command instructing the server to connect to a port on the client one higher than the client s control port. This is the client s data port. 3. The server sends data to the client from server port 20 to the client s data port. Passive Mode 1. The FTP client initializes a random port higher than 1 023 as the control port and initializes the port one higher than the control port as the data port. 2. The FTP client sends a PASV command instructing the server to open a random data port. 3. The server sends a PORT command notifying the client of the data port number that was just initialized. 4. The FTP client then sends data from the data port it initialized to the data port the server .