Network Address Translation – NAT

 In this video I’ll talk about network address translation how it works and why we need it but first let’s hear a bit of backstory where do these little baby IP addresses come from anyways back in the late 70s a couple of clever researchers were thinking about how long IP addresses should be why is this important you have to consider that in every data packet we have to encode the sender IP and the receiver IP if we only reserved a few bits to encode an IP address we can keep packet sizes small however will also have few IP addresses to end out to people if on the other hand we choose to have lots of bits to encode an IP address we’ll have lots of IP addresses to hand out but we’ll also have huge packets which increases the communication overhead so as you can see there’s a bit of a trade-off going on here you can either have small packets or lots of IP addresses they ended up deciding that they’d go with 32 bits for an IP address that left them with roughly 4.2 billion IP addresses you have to consider that back then the Internet which wasn’t even called the internet back then was just a toy for academics it was definitely not meant for regular people like you and me and that’s what’s known today as IP version 4 now hang on a second let’s do a bit of math here we have 4.2 billion IP addresses to hand out but we also have seven billion people living on this planet as of 2012 and also consider that each of those seven billion people may want to use multiple devices to access the Internet so yeah we have a bit of a problem on our hands what are we going to do about this now because clearly having only four point two billion machines on the Internet is not going to be an option we have to figure something out and there is a fix but it’s as you’ll see a bit of a workaround let’s keep it at that there is in fact a way that we can keep using IP version 4 and get more machine onto the Internet here’s a typical home network setup this is John’s laptop this is Jane’s laptop and this is Jane’s smartphone they’re all connected to the home router that home router in its turn is connected to the Internet now when you register with your ISP you get to use an IP address that is accessible throughout the internet and that’s assigned to your home router so in this case we have 12 13 14 dot 15 assigned to the home router this is a public IP address that theoretically anyone on the internet can send packets to our laptops and smartphone also get their own IP address but instead of getting one of those 4.2 billion public IP addresses they get a private IP address that private IP address is designed to them by the home router and is not accessible from the Internet now if these IP addresses look familiar to you that’s probably because they are this is one of two ranges that are reserved for private networks all addresses starting with 192 dot 168 are reserved for private networks as well as all addresses that start with the number 10 you can use these IP addresses without having to worry that some random web server already uses that IP address they can because it’s specifically reserved for private networks now let’s consider the connection from Jane’s laptop for a second Jane’s laptop has an IP address of one ninety two dot one sixty eight dot one dot three here’s an example scenario where Jane’s laptop wants to know what the current weather is in taxes and to get this information it will try to contact the server at 14:30 2010 to get that information to reach this web server james laptop will have to go through the home router through the internet and finally then arrive at the web server first Jane’s laptop has to send out a pack requesting the information it says what’s the current temperature in San Antonio Texas now the message here isn’t really important the web server should know what to do with that and we won’t get into that right now but what you should know is that there is a source IP address here with the source port and a destination IP address with a destination port in this case it’s port 80 which is frequently used for HTTP traffic now let’s send this package on its way shall we when it arrives at the web server the web server will process the incoming requests and try to formulate a reply the reply packet will be addressed to the sender but in this case because it was sent from 192 dot 168 at 1.3 it will not be reachable because it’s a private IP address so what do we do now let’s rewind that tape for just a second Jane’s laptop sends out a packet requesting the current temperature and San Antonio taxes it gets sent along to the home router but instead of sending it right over the Internet the home router does something that’s a little bit sneaky when that packet re-emerges you’ll notice that the source IP address has been changed this is not the only thing that the home router does in addition it also creates an entry in the net forwarding table now we’ll talk about net forwarding table again in just a second but for now all you have to remember is that this thing allows us to know which packets are to be sent in Jane’s laptop when they come in and which packets are to be sent to that smartphone or John’s laptop moving right along the packet travels over the Internet and arrives at the web server the web server will now create a reply packet and will notice that the destination IP address is no longer private no instead it’s sent to the public IP address of our home router on that specific port now that our home router has received the response it’s time for another tiny lie it creates a packet that looks as if it was addressed from the web server directly to Jaymes laptop which is not what happened but that’s all Jane’s laptop needs to know because it doesn’t care about how it got there it just wants to get the packet and get this over with but how does it know where to send that packet it just received the answer is in the net forwarding table we just received that packet on port 24 604 and that means if we look in our net forwarding table that we have to change the IP address on the private side to 192 dot 168 1 3 on that specific port over there in the forwarding table Jane’s laptop receives the packet and the rest is history Jane’s laptop doesn’t have to care about any of that NAT stuff the home router completely handles it for Jane’s laptop and it is completely transparent so James laptop doesn’t have to worry about public and private IP addresses no this is something through router fixes and James laptop doesn’t have to worry about it of course ideally every computer would have its own public IP address which is why we’re slowly but well actually just slowly moving over to IP version 6 remember how IP version 4 had 4.2 billion IP addresses at its disposal well IP version 6 has a lot more and I do mean a lot seriously try noting 34 and then followed by 37 zeros and that’s how much ipv6 addresses we have approximately that is a lot if we can give every computer its own public IP address we won’t be needing network address translation anymore this means that routers can once again focus on their core business which is routing packets not doing awkward switcheroo tricks now I can hear you saying this is nice and all but why haven’t we switched yet this IP version 4 thing is a mess we only have 4.2 billion IP addresses and that is clumsy as hell well it’s not that simple we have a lot of Internet infrastructure that was built up over the years and they all work with IP version 4 so we have laptops desktops home routers but also stuff on the ISP side and web servers and they all work with IP version 4 all these devices need to have hardware and software that supports IP version 6 before we can entirely switch over and that is a huge task just to give you an idea the IP version 6 standard was finalized in December of 1998 and yet here we are 14 years later and was still running on IP version 4 you can see how this is one hell of a task alrighty if there’s anything you’d like to see explained please do let me know in the comments below thanks for watching and I’ll see you next time