IP Concepts

In this module we are going to cover various aspects of IP. We are going to start by looking at oneof the most common protocol stacks: The OSI protocol stack and look at how communications isbroken down into seven core areas. We are then going to compare the seven layer OSI stack withthe TCP/IP protocol stack. Then, we are going to cover numbering systems and see how to translatebetween decimal and binary.
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IP Concepts IP Concepts Security Essentials The SANS Institute IP Concepts - SANS ©2001 1Hello, welcome to IP Concepts. This is one of two modules on the subject of TCP/IP. Since theTCP/IP protocol suite is fundamental to any network that is connected to the Internet, in order tosecure TCP/IP based networks, one must understand how they work and why they work. In thismodule we are going to start by looking at the IP protocol and then in the second module we willcover the other protocols that reside in the TCP/IP protocol suite.Lets jump in and start looking at how IP works. 2-1 Objectives • Protocol stacks • Numbering systems • Packets, how is data transmitted • IP • Addressing IP Concepts - SANS ©2001 2In this module we are going to cover various aspects of IP. We are going to start by looking at oneof the most common protocol stacks: The OSI protocol stack and look at how communications isbroken down into seven core areas. We are then going to compare the seven layer OSI stack withthe TCP/IP protocol stack. Then, we are going to cover numbering systems and see how to translatebetween decimal and binary. Since humans operate on a base 10 numbering system and computersoperate on a base 2, it is important that you understand how to translate between the two systems.Then we will cover how data is put into packets and sent across the wire. IP, which is thefundamental protocol of TCP/IP, will then be covered in-depth, looking at various aspects of theheader. We will finish the section by looking at IP addresses and how they are broken down into twopieces: A network and a host portion.Now lets start by looking at what a protocol stack is. 2-2 Protocol “Stacks” • Divide network communications into layers • Each layer provides service to the layer above and receives service from the layer below • Divide task of communication into pieces for easier implementation IP Concepts - SANS ©2001 3 Having all these protocols is nice, but how does information actually get from one place to another? In order tounderstand how this happens we need to start by discussing protocol stacks. And to discuss protocol stacks, we needa good real-life example. Imagine, if you will, a five-story apartment building. This building, however, is very special (or very strange,depending on your point of view). First of all, the really important things happen on the top floor, floor 5. Second, theonly way the people on the fifth floor can get anything done is by asking the people on the floors below them to do it.For example, the people on the fifth floor want to make dinner. They tell this to the people on the fourth floor. Thepeople on the fourth floor figure out that dinner requires a soup, salad, main course, and dessert. They tell this to thepeople on the third floor. The people on the third floor decide that the courses will be onion soup, a garden salad, beefstew, and apple pie, and they tell this to the people on the second floor. The people on the second floor figure out whatingredients will be needed for this dinner (for example, chicken broth, lettuce, vegetables, beef, etc.) and give thisinformation to the people on the first floor. The people on the first floor actually go to the store, buy all the ingredients,and bring them back to the apartment building. Once the ingredients are purchased, the process goes in reverse. The first floor gives the raw ingredients to thesecond floor. The second floor checks that all the ingredients are there and then hands them off to the third floor. Thethird floor prepares the various courses by making the soup, tossing the salad, cooking the beef, and baking the pie.Once all this is done they hand the food off to the fourth floor. The fourth floor people package all the food up intonice courses and bring it up to the fifth floor residents so they can eat a delicious meal. Well, in essence, that’s how protocol stacks work. Protocol stacks divide network communications into differentlayers, like the floors in the apartment building. Each layer in the stack works on the packet in different ways. Somelayers make sure the packet has all the information it needs, some layers make sure the packet is ready for anapplication to work with, and some layers make sure the packet gets onto the network properly. Each layer worksdirectly with the layer above and below it, just as in the apartment building example. As packets are passed from onelayer to the next, each layer examines or modifies the packet in some way. Once the packet has reached the “groundfloor” of the network it is sent to its destination. The use of protocol stacks in network communications makes the task of implementing protocols much easier. Bymaking communications more modular, a service, process, or application need only concern itself with the layers itneeds, leaving the other layers to someone else. 2-3 The OSI Protocol Stack Application Layer 7 Presentation Layer 6 ...