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May 13
Client / Server Overview

Example of Client / Server software

Microsoft OutLook email (client) communicating with a remote Microsoft Exchange (Server) using standard port numbers for SMTP and POP3 Client processes don't use well-known or registered ports. Instead, each client process is assigned a temporary port number for its use. This is commonly called an ephemeral port number.

ephemeral: “short-lived; existing or continuing for a short time only.”

Client processes contact server processes using the server's host IP address and registered port number. To know where to send the reply, the server must know the port number the client is using. This is supplied by the client as the Source Port in the request, and then used by the server as the destination port to send the reply. The graphic below, Sample IP Packet, shows the structure of a request.
 
April 01
Network Engineering Primer for 3rd Graders

This primer is for the newbee. Someone who could also be a software or systems engineer or an administrator that's looking for a simple way to ease into a complex and confusing subject matter. Reading material that will give enough meat about networking concepts, enough to keep up when in face-to-face or group discussion with a Network Engineer. When Network Engineers talk, they'll assume you know the intricacies and semantics of the 7-Layer OSI Model and the TCP/IP protocol stack.

Because I knew in advance that this topic can cause mental blockages and nose bleeds, I'll write this at the 3rd grade level using concepts that a 3rd grader can easily understand. It's not intended to offend or insult you but, let's face it, if you're reading this primer, it's probably because the grown-up adult web pages at Cisco made you sleepy and gave you a nose bleed at the same time.

So, let's get familiar with Network Engineer speak. The OSI Model describes everything about the network by breaking it down to 7 layers. I'll familiarize you with key parts of the OSI Model that will most likely be brought up in a typical conversation with network engineers.

The 7 Layer OSI Model

Layer 1. The Physical layer. Inside Daddy's computer and Mommy's cell phone is a little device. You can call him NIC. NIC is a cute little guy and his name stands for Network Interface Card. He has a job that's called, an adaptor. NIC gets Daddy's PC and Mom's SmartPhone on a highway called, WiFi or the LAN. If Daddy's PC connects to a box at home to get Internet then NIC, the cable and the box is all part of Layer 1.

Layer 2. The Data Link layer. The guy that works here is like the guy at the post office that took your letter for Grandma. He's like that box in Daddy's office that's blue and has 4 or 8 ports with wires hanging off of it. They're called hubs or switch. Well, all the guys that are in this layer are handling the transmission of data. The guy at the post office took grandma's letter and put it into a special package called a "frame". Not a picture frame silly. Well, okay, something kinda like that.

Layer 3. The Network layer. Remember that nice old man working at the train station? The old dude that inspected your train ticket, making sure you and me get on the right train going to grandma's house? That box at home in Daddy's office with the wires hanging out of it and connects to another box that has a wire going into the wall, remember this box? It's the box that funny looking guy from the company called Cox came over to hook up? Well, this is the real box that gets Daddy's computer onto the Internet. At work, we call it a "Router". Except that the Routers we have at work are the size of a small fridge! Remember that time Uncle Jack had to stay home from work for 2 weeks? It's because one of these big routers fell on his foot! Anyhow, Router devices no matter what size they are, reside in layer 3.

Layer 4. The Transport layer. Remember the guy who drove the train to Grandma's house? Your letter to Grandma also rode on a train by itself. Scary riding alone huh. Don't worry. This guy works with another guy making sure Grandma's letter made it to her house.

Layer 5. The Session layer. Did you know the train guy has a big boss who ensures that each train station, railroad track, track signal lights and stuff are all connected so that you don't have to worry about Grandma's letter not getting to her house?

Layer 6. The Presentation layer. What? You're worried that no one will understand what you wrote on the envelope of Grandma's letter? Don't worry. The guy that works at this layer will make sure the people at the other train station near Grandma's house will be able to understand your scribbles and scratches.

Layer 7. The Application layer. Remember when Mommy let you use her computer to visit Barney the Dinosaur's web site? The Browser was able to do this by speaking a special language they refer to as a protocol. They call it HTTP. Yep, pronounced, huttup. Remember when Mommy accidentally downloaded a virus on Daddy's computer? This bad software spoke to its friend somewhere on the other side of the planet using the FTP protocol and that's what it used to download another bad friend to break into Daddy's bank.

TCP/IP

Remember when you got silly and ran into your sister's room, and you annoyed her because she was talking to her school friend, and then they started talking in code you couldn't understand? She said something like, "Iway ishway Iway ouldcay urntay isthay ittlelay erptway intoway away utecay ittlelay amsterhay" Well, that's special code talk between girls called Pig Latin. She told her friend that she wishes she could turn you into a cute little Hamster. I know, right?! Don't tell her I told you that. Anyhow, all those guys in the 7 layer OSI model use a special code talk too, a protocol called, TCP/IP.

This protocol comes in two parts. The first is called Transmission Control Protocol and the second, Internet Protocol. A secret group in the United States Government called DARPA paid to have these two guys created to help them rule the world with their sinister and deadly free government cheese program! Mwuuhahaha!

That letter you wrote to Grandma might be simple to you but, it takes a lot of hard work to make sure it's packaged up tight and delivered right. Your little letter has to be put inside of a thing called a packet. This packet has all kinds of information in it, and it looks like this.

Figure 1. Source: http://technet.microsoft.com/en-us/library/Cc700820.fire01_big%28l=en-us%29.gif

See that little box with the word Message in it? That's where Grandma's letter is. That box with Source IP Address has Daddy's computer's address in it and Grandma's computers's address is in the Destination IP Address box. The boxes with Ports in it? Well, it's kind of like the shipping ports where we went to pick up your pet Gorilla. That was the port of Lake Titti-Caca and other ports around the world know it by its name and ID. Just like a home address! Because it was a letter to Grandma, it had to use a port ID just for that purpose. In the case of Grandma's letter, since we sent it using E-mail, it used port number 25. Cool huh?!

So, what do you think? Should we review?

  1. You went on to Mommy's laptop, and connected to the Internet – Layers 1 through 5 in the OSI Model
  2. You fired up Google Chrome, went to GMail, typed up your Christmas list and sent it to Grandma – Layers 7 through 1 in that order
  3. The message you wrote in Norwegian was packaged up, addressed and went through the wire in something that looks like a box frame shown in figure 1
  4. TCP/IP is the protocol language all of them spoke while sending and delivering grandma's letter

Good job! Look at you. Smarty Pants! So, let's review again what's the first thing you're going to buy with your first paycheck from a Network Engineer job? Right! Daddy's Jet Ski and trailer! Good job!

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About Me
I am currently a Developer and Build Automation Engineer based in Northern California. I have had a career in Information Technology since 1989. Previous positions I've held with large corporations in the San Francisco Bay Area helped spur my professional and technical development across a variety of platforms, operating systems, programming languages, voice and data network topologies and communications protocols. My work history demonstrates having gone full circle in the myriad of IT departments serving in support, lead, management and consulting roles. Throughout the span of my career, I've long maintained a keen interest in extending software applications to support a high level of end-user productivity.

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