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In some sites , It is given that tcp/ip has 4 layers,network access,internet,transport,application. But in some other references , it is given that it has 5 layers (instead of network access, physical and link). What is the standard actually??? Somebody knows well tell me pls.

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What does your own research suggest? This is now the second question you've posted which shows no research effort at all. –  Dave Rook Sep 18 '13 at 13:55
    
@DaveRook See,I haven't posted without any try. I am googling with my team with no luck. We know both 4 and 5 layers are same. But we want to be clarified with the exact standard ! –  Karthik Sep 18 '13 at 14:01
    
The term you are looking for is "OSI Model" –  Justin Pearce Sep 18 '13 at 14:12
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@JustinPearce, I don't think it is. The OSI Model has neither 4 nor 5 layers, it has 7. See the image in this answer for a good comparison. –  heavyd Sep 18 '13 at 14:19
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Just so everyone is clear, the OSI Model and TCP/IP Model actually have differences outlined on Wikipedia and in several other places. –  dotVezz Sep 18 '13 at 14:24
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2 Answers

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It seems like a combination of circumstances here has led to some confusion. Just so everyone is clear, The TCP/IP Model has a slightly different set of layers when compared to the OSI Model.

To be clear, the OSI Model's top three layers - that is: Application, Presentation, and Session - are essentially collapsed into the Application layer in TCP/IP. Additionally, the bottom two layers - Physical and Data Link - are combined into the Network Access layer for TCP/IP.

Therefore, there are 4 layers in the TCP/IP Model. Specifically, they are the Network Access Layer, Internet Layer, Transport Layer, and Application Layer.

You can find some (Difficult to read) technical information in RFC 1122, and some better-presented educational information provided by the University of Pittsburgh.

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I believe you are conflating the DOD\DARPA stack for IP with the OSI model

see here for how the layers of each model map to each other.

http://netcert.tripod.com/ccna/internetworking/dod.html

keep in mind, there are aspects of protocol modeling that are purely conceptual, and don't necessarily mimic analog reality. for instance in modern networking, the osi L1 and L2 are implemented in the network card's circuitry and in its driver code, but there's no good way to tell where one ends and the other begins. afterall the task of sending a media-compliant frame across the network cannot be separated from the task of constructing a 802.3 frame with all the correct data structures.

The layers are not necessarily concrete (especially when viewed from app perspectives), and as such there may not be a "correct" answer. A protocol can have as many layers as as you choose to slice it up into, regardless of the architecture of the code and the circuitry.

Models are often just how you think about a problem, and in this case, both models are valid for different analytic purposes.

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Oh well darn. Your answer already says everything that mine does. Good show. I should have read a bit deeper before posting my own. –  dotVezz Sep 18 '13 at 14:38
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