Interaction Between OSI Layers

Imagine a Web browser displaying a Web page that it received from a Web server. Before that happened, the browser somehow interacted with the software implementing other layers of TCP IP on the client computer, causing a request to flow to the server. Likewise, the broswer application somehow communicated with the Web server application, telling the server what Web page the browser wanted to display. A fancy way to describe these two ideas that is interaction between OSI layers. The process of...

Default Routes and the ip classless Command

When a router needs to route a packet and there is no route matching that packet's destination in the routing table, the router discards the packet. Default routing lets the router forward the packet to some default next-hop router. Default routing is that simple However, two configuration options for default routing make it a little tricky. Also one other option changes the algorithm of how the router decides whether there is a routing table match, which affects when the default route is used....

Answers to the Chapter 3 QA Section

1 Name the seven layers of the OSI model. Application (Layer 7), presentation (Layer 6), session (Layer 5), transport (Layer 4), network (Layer 3), data link (Layer 2), and physical (Layer 1). Some mnemonics to help you recall the names of the layers are All People Seem To Need Data Processing (Layer 7 to 1), Please Do Not Take Sausage Pizzas Away (Layer 1 to 7), and the ever-popular Pew Dead Ninja Turtles Smell Particularly Awful (Layer 1 to 7). 2 What is the main purpose(s) of Layer 7 Layer 7...

Avoiding Loops with Distance Vector Protocols

Routing protocols carry out their most important functions when redundancy exists in the network. Most importantly, routing protocols ensure that the currently best routes are in the routing tables by reacting to network topology changes. Routing protocols also prevent loops Distance vector protocols need several mechanisms to prevent loops. Table 7-8 summarizes these issues and lists the solutions, which are explained in the upcoming text. Table 7-8 Issues Relating to Distance Vector Routing...

Destination Unreachable ICMP Message

The ICMP unreachable message is sent when a message cannot be delivered completely to the application at the destination host. However, packet delivery could fail for many reasons, so there are five separate unreachable functions (codes) using this single ICMP unreachable message. All five code types pertain directly to some IP, TCP, or UDP feature and are better described by using Figure 6-19 as an example network. Assume that Fred is trying to connect to the web server, called Web. (Web uses...

Four Steps to Answering IP Addressing Questions

You must master IP addressing and subnetting to succeed as a network engineer. To pass the CCNA exam, you must at least be able to answer a few questions about subnetting. For most networking jobs, the ability to think about IP addresses and quickly decipher the structure and meaning of the address is a prerequisite for the job. The exam will test your abilities with questions that go something like this Given a network number and a mask, how many subnets are there, and how many hosts are there...

Comparison of LAN Segmentation Using Bridges Switches and Routers

The CCNA exam tests your ability to compare networks that use bridges, switches, and routers to separate LAN segments. The basic workings of a bridge and a switch have already been covered in this chapter. To compare these three options, first routing logic is covered. Afterward, comparisons will be made among the three options. Routing is covered more fully in other chapters. For comparison, a router separates two segments in Figure 4-14. The client initiates the same example flow that was...

IP Grouping Concepts and Subnetting

The creators of the Internet realized the impracticality of the original network-numbering conventions early on. Computing history shows many examples of people being unable to conceive the idea that computing technology would grow as fast as it has. Needless to say, the Internet would have run out of Class A, B, and C networks long ago if additional addressing features had not been created. Subnetting provided the first significant addressing feature that conserved the global IP address space....