The OSI & TCP/IP Models

The OSI (Open System Interconnect) model is a product of the International Standards Organization (ISO). It consists of seven layers that define how information is transferred across networks. The layers, from lowest to highest, are the physical layer, datalink layer, network layer, transport layer, session layer, presentation layer, and application layer. Knowledge of this model is required for most any networking test out there, and is often used during job interviews to see how well one understands networks. One way to remember these layers is with a mnemonic. If you google the OSI layer, you will find a long list of mnemonics that are meant to help! Her are a few:

"People Design Networks To Send Packets Accurately"
"People Don't Need This Stuff Presented Anyway"
"People Don't Need To Study Protocol Analysis"
"Phil Donahue Never Televises Sick People Anymore"
"Philys Did Networking Till She Passed Away"
"Please Do Not Take Sales-People's Advice"
"Please Do Not Tell Secret Passwords Anytime"
"Please Do Not Throw Salami Pizza Away"
"Please Do Not Throw Sausage Pizza Away"
"Please Do Not Touch Steve's Pet Alligator"
"Please Don't Network These Stupid People Again"
"Programmers Dare Not Throw Salty Pretzels Away"
"Programmers Do Not Throw Sausage Pizza Away"

If you want to go the other way, these are from top to bottom!

"A Pathetic Silly Trick Never Does Please"
"A PC Sees The Network During PowerUp"
"A Perfect System That Never Did look Perfect"
"Active Penguins Seek the Nearest Deep Pool"
"All Parents Should Teach New Dads Parenting"
"All Penguins Stand Too Near Deep Pools"
"All People Seem To Need Data Processing"
"All People Seem to Need Dominos Pizza"
"All people should teach networking daily please"
"All People Studying This Need Drastic Psychotherapy"
"All Pizza Seems To Need Double Pepperoni"
"All Pre-School Toys Need Durable Parts"
"All Pretty Serious Teenagers Never Do Physics"
"American Presidents Should Try New Dating Practices"
"And Please Send Them New Delhi Pie"
"Angus Prefers Sausages To Nibbling Dried Pork"
"Apply Proper Sense To Network Data Path"
"APS Transports Network Data Physically"
"Australian Post Sucks They Never Deliver Parcels"

Personally, I have more trouble remembering the mnemonics!! At least the layers make sense to me!! Perhaps that comes with understanding how the layers are meant to function together.

The TCP/IP model is probably a more useful model, and is often shown beside the OSI model to compare it's layers. This model was first defined by DARPA in a pre-Internet description of the ARPANET. The TCP/IP model has only 4 layers: the Network or link layer (which consists of the physical & datalink layers of the OSI), the IP layer (which compares to the OSI network layer), the TCP layer which compares to the transport layer of the OSI, and the Application layer which spans the session, presentation and application layers of the OSI.

Since the IP protocol suite are the primary protocols used on the Internet, learning and using the TCP/IP model is critical to understanding the Internet architecture.

For additional information, there are great descriptions of these models on Wikipedia. Also see RFC-1122 and RFC-1123 for a discussion of the Internet layer protocols and architectures.