Sausage Pizza, Big MACs, and IPs

Before I dive into this week's subject at hand, I want to say that I am taking a crawl, walk, run approach to this blog, so if you are an absolute computer wizard, I appreciate your patience while we discuss more basic principles. Plus, if you are like me, I enjoy approaching old content from a new angle. It helps me reinforce what I've already learned. That said, I promise that we will tackle more complex topics in the future. To those that are new to computing, I encourage you to go back to the beginning of this blog and read through. Ok, now on to this week's post!!

Last week, I covered mostly the 1st layer of the OSI model, the physical layer. I didn't mention the model in that post, but it was developed by the International Standards Organization (ISO) to describe how computers communicate with each other. There are seven layers, and they look something like this:

In school, I learned a very useful acrostic to remember it. Reading from the bottom up, it went something like this:

Away - Application Layer
Pizza - Presentation Layer
Sausage - Session Layer
Throw - Transport Layer
Not - Network Layer
Do - Data Link Layer
Please - Physical Layer

We will work our way up in future posts, but for now, we will focus on Layers 2 and 3. These layers deal with how information is routed and addressed. A computer, sometimes referred to as a node on the network, must start at layer 7 (Application Layer) when sending information and at Layer 1 (Physical Layer) when receiving information. Layer 7 is where humans interact whereas Layer 1 is where variances in voltages and sometimes light impulses are represented as the ones and zeros of digital communication. Everything in between is basically processing, much like if you were to address and send a letter in the mail to your aunt in Albuquerque. You would need to include a to and from address, your aunt's name, special instructions for delivery, and maybe even a request to your aunt to respond to let you know she received it. Digital information is no different, except that we call it different things based on what layer it is in. Layer 1 is bits. Layer 2 is frames. Layer 3 is packets, and etc.

Unlike a tangible letter though, digital information, or packets (Layer 3) is often split into many tiny pieces that are numbered then sent along different routes and reassembled at the destination. Remember the Wonkavision from Willy Wonka and the Chocolate Factory? It's exactly like that, but without the shrinking effect.

Alright, so that's how information is sent, how does it get to the right place? Well, that is fairly simple. Just like your physical home address, you also have a digital address, know as your IP (Internet Protocol) address. There are two versions of IP addresses in existence today. The first is IP version 4 (IPv4) which is more prevalent. The second is IP version 6 (IPv6) which is the eventual replacement for IPv4. The problem with IPv4 is that there are only 4 billion addresses for a global population of 7 billion, and each device on the internet (which is far greater than 7 billion by the way) needs an IP address. To resolve the issue, the internet's caretakers came up with IPv6 which has over 340 undecillion addresses. That is more unique addresses than every single grain of sand on earth!! But, some old things die hard, and implementing IPv6 is costly, so the life of IPv4 was extended by Network Address Translation (NAT). NAT can be thought of as an apartment building. The outside of the building contains a public address while the inside of the building contains apartment numbers. Likewise, your router receives a public IP address from your ISP, but behind the router is your private network, and each device on that network receives a private IP address from your router or whichever server assigns IP addresses. It is your router's job to take packets coming from the internet and route them to the correct device on your network.

This is where Media Access Control (MAC) address comes in. Unlike IPs which are usually handed out and returned, MAC addresses are hard coded into the device by the manufacturer, and each MAC is unique. Your router keeps a list of IP addresses and corresponding MACs so that it can do its job, but sometimes it has to update this list and learn new MACs. To do this, it uses Address Resolution Protocol (ARP). ARP is basically like yelling into a crowd "Who lives at 253 East Main Street?" If Jim responds, then you know that Jim lives at that address, and you will know how to deliver letters addressed to Jim. In a future post, I will discuss Domain Name System (DNS) which resolves IP addresses to domain names like www.google.com. Since humans would have a hard time remembering some arbitrary number like 172.217.9.46 (the IPv4 address), we've created a system whereby that address is resolved to www.google.com. In the real world, you may not know your local Walmart's street address, but if you asked a friend to take you there, they probably could. DNS is similar, but to be clear DNS is different from MAC addresses and ARP. ARP resolves MACs to IPs and vice versa while DNS resolves human readable domain names to IPs.

There is much more I'd like to talk about, but I'd better stop here for now. Next week, I'll discuss TCP and UDP, two of the more well known and common protocols. We'll also discuss commonly used port numbers. I'll see you then!!

References:

Welcome to the Net training video