Understanding Networks

Charles Zangla

Network Models

• Before I can explain how connections are made from across the country, I would like to provide you with a knowledge of network models and how they can assist your understanding of the way networks well.. Work!

Network Models

• There are two predominant network models, but I am only going to focus on one, the OSI model.

• The OSI model consists of seven layers:– Physical– Data-Link– Network– Transport– Session– Application

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Physical Layer

• The physical layer consists of all the components that move data between computers(cables, hubs)

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Data-Link

• The key element of the data

Link layer is the NIC.• The NIC is the interface that

allows your PC to talk to the network• The NIC also constructs frames which

encapsulate data with important information like where the packet is going and who it came from.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Network Layer

• This layer is where the real magic starts to happen. Routers like the one below operate on this layer. IP addresses are assigned on this layer. And don’t worry, we’ll get more in depth later.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Transport & Session Layer

• Transport Layer is responsible for assembling and disassembling packets as they come and go down the line (some packets need to be broken up before they are sent down the line because they are too large).

• The Session Layer handles all the sessions for a system. It initiates sessions, accepts incoming sessions, and opens and closes existing sessions. A session is nothing more than a connection.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Presentation & Application Layer

• All of this wouldn’t mean anything without the applications that allow us to utilize network connectivity. This layer refers to the code behind software such as Firefox, not the application

itself.

Now, let’s get back to the question at hand.A user types our URL in a web browser in, say, California.

The web browser makes a request to the operating system(OS e.g. Windows 7) for the www.yourcompany.com’s server IP address. The OS does a DNS lookup to match the name to the IP and sends the IP to the browser. The browser initiates a TCP connection through HTTP on port 80 with the remote web server. A request is sent from the PC to our Web server for our homepage. Our web server encapsulates our homepage into a series of packets which are then framed and sent to the PC to be reassembled and displayed as HTML by the browser on the PC. Now, let’s break this down..

In order for the web browser to connect to the remote web server it needs the IP address. An IP address is a unique set of numbers assigned to each NIC on a network. It is in the form of four octets with each ranging from 0 to 255 (Ex: 192.168.1.10). The OS uses DNS to find the IP address. DNS (Domain Name System) is a system that maps Host Names such as yourcompany.com to corresponding IP addresses.

IP Address?

DNS Server

IP?

192.168.1.10

192.168.1.10

The PC then sends a connection request to the server’s IP via TCP (Transmission control protocol) on the network layer of the OSI model. TCP is known as the connection oriented protocol because when you send a packet using TCP, an acknowledgement is sent from the receiving system. In other words, it’s the only way to ensure a message was received.

TCP/IP applications, like Firefox, use TCP/IP protocols to move data back and forth. Web browsers use HTTP (Hyper Text Transfer Protocol) to communicate on port 80. Certain ports are assigned to certain applications. This number is stored in the TCP headers of an IP packet so that the operating system knows for which application the packet is intended.

Recipient’s MAC address Sender’s MAC address Data FCS

Generic Frame

Source Port Destination Port Sequence Number ACK

TCP Header

The frame is prepared by the NIC, which is sent specific instructions from the OS on what to put in the frame (sender’s MAC, recipient’s MAC, etc.). The packet within the frame was prepared previously by the TCP/IP application requesting network communication. Among other things the packet consists of the actual data sent between computers. The frame is sent from the PC’s NIC to the Default Gateway, which is the network’s router or switch. The router looks at the recipient IP and decides whether it can route the packet to the destination or if it needs to forward the packet to the next hop router (the next router up in the routing table). Once the packet reaches the Destination IP’s gateway , the IP address in the header is replaced with the recipient’s MAC address. The MAC address is a unique ID, Similar to the IP address, that is imprinted on every NIC ever made.

The receiving NIC gets the packets on Layer 2 and the Transport Layer protocols assemble the packets. The application layer TCP/IP software listen on their respective ports for data. The client PC in California’s Browser hears data incoming on port 80. It knows that this is HTTP and retrieves it from the NIC. The client then displays the HTML webpage.. THE END!!