OSI material Flashcards
Familiarize with levels, equipment, ports and protocols
What does OSI stand for and why is it important?
Open Systems Interconnection was defined in the early 1980s by ISO (International Organization of Standards, ISO 7498). It standardized network protocols – which had been largely proprietary to that point. As it stands this is a model or framework that’s abstract and theoretical.
What are the layers? How are they numbered?
Numbered one to seven from bottom to top, from Physical to Application. Each layer communicates with its peer layer on another device.
Explain how data is changed (encapsulated, i.e.) going through the OSI model from application to physical layers.
Message entering the stack at Application Layer is DATA (stream). This state is maintained through Presentation and Session layers until it hits the Transport layer – complete with (usually) header material – where it’s called either a SEGMENT (for TCP protocols) or a DATAGRAM (for UDP protocols). It gets more encapsulation in the next (third) layer, the Network layer and is called a PACKET. By the time it goes to the next (second) layer, the Data Link layer, it’s now probably got footers, too – and is called a FRAME. This is converted to bits for communication from physical to physical layer at that point.
How is the TCP/IP model related to the OSI model?
TCP/IP is a little older (DARPA-related) and has only four layers instead of OSI’s seven. It takes the OSI first two layers – Physical and Data Link – and combines them into the “Link” layer, AKA “network access layer.” Next, the OSI third layer (“network”) is called the “Internet” layer for TCP/IP. The fourth layers, “Transport,” is also called “Transport,” but may also be called the “Host-to-Host” layer for TCP/IP. Finally, the last three OSI layers – Session, Presentation and Application – are combined into the TCP/IP “Application” layer.
In TCP/IP, what’s a port?
Address numbers for the “Transport” layer that get used in the TCP and uP protocols. AKA a “socket,” the number for any port is agreed to between links. A port allows “a single IP adress to able to support multiple siumltaneous communications, each using a different port number” (Stewart, p. 439). All port numbers are 16 digit binary numbers for a total allowable number of 2^16 or 65,536: 0-1023 are “well-known” or “service” ports: 1024 to 49151 are “registered software ports;” and 49152 to 65535 are “random, dynamic or ephereral” ports.
What’s TCP Port 23?
Telnet: “Terminal emulation Network” supporting remote connectivity for executing/rnning commands and applications but NOT THE TRANSFER OF FILES.
What’s TCP Ports 20 and 21?
FTP: “file transfer protocol” is a network app tht DOES SUPPORT EXCHANGE OF FILES that requires anonymous or specific authentication.
What’s UDP Port 69?
TFTP: “Trivial File Transfer Protocol” is a network app that supports exhange of files that DOES NOT REQUIRE AUTHENTICATION.
What’s TCP Port 25?
SMTP: “Simple Mail Transfer Protocol” is used to transmit email messages from a client to an email server and from one email server to another.
What’s TCP Port 110?
POP3: Post Office Protocol used to pull email messages from an inbox on an email server down to an email client.
What’s TCP Port 143?
IMAP: Internet Message Access Protocol is like POP3 only more secure.
What’s UDP Ports 67 and 68?
DHCP: Dynamic Host Configuration Protocol uses 67 for server point to point response and 68 for client request broadcasts. It enables centralized control of network addressing.
What’s TCP Port 80?
HTTP: Hypertext transport protocol transmits web page elements from a web server to web browsers.
What’s TCP port 443 (for HTTP encryption)?
SSL (secure sockets layer) is VPN like security protocol operating at the Transport layer. Can support HTTPS or any Application layer protocol communications.
What is TCP Port 515?
LPD: “Line Print Daemon” spools print jobs and sends print jobs to printers
