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October 27, 2020

Hey, what’s your address?

No, I mean your local address.

Oh, you geeky nerd!!! I mean your physical address.

– Saved from

Yesterday I talked about how your internet address works. I related it to your street address. The pieces of your internet address tell computers how to find your computer. Or how your computer can find others. (It wasn’t as boring as it sounds.)

Now lets talk about your local address. Or your “home” address, It’s also called the loopback address. But what is it?

Well, when the rest of the world talks to your computer, they talk to your IP address. But, what if your computer wants to talk to itself? Programs inside the computer can already communicate, of course. When you save a document, you can open it in another program. But, when you open an email, you computer is talking to an internet server. Just as you know if you can’t get reception, you can’t get to your new email.

Well, your computer has the ability to “pretend” it is talking to the internet. If I’m writing a network based program, I might want to use my computer to simulate both a client and a server. In that case, I would send a network connection to That request would “loopback” to my own computer.

The loopback address is a special address. There are other “special” network addresses as well. Here are a few: –……. Current Network –….. Private Network –… Shared space used for ISP for NAT – .Used by DHCP servers – …Private Network – …….IETF Protocal Assignment – …….Assigned as TEST-NET-1 – …Reserved – .Private Network – …Private Network used for testing subnets – .Assigned as TEST-NET-2 – …Assigned as TEST-NET-3 – …Used for IP multicast – …Reserved Reserved for “Limited broadcast”

And of course, your loopback address –

Okay, who cares, right? I can hear some readers now, “Rodney, you normally write more entertaining posts than this. But, you’re starting to lose me.”

Stay with me one more minute.

Everything on the internet has an address, right? We’ve talked about unique addressing. And just as two houses in the same town can’t have the same address, no two computers on the internet can have the same address.

Have you ever lived in an apartment? You know, one that has dozens or hundreds of units. Suppose your apartment was number 214. There’s no one else in your building that has apartment 214. But, does anyone else have apartment number 214 if they live in a different building? Of course.

The interent is like that. If you look at that list of “special” addresses you see several that are for “private network.” Those ranges of addresses are like apartment numbers. Your own private network can assign any and all of the addresses in that range. . .and so can the next company down the road.

But, how to we keep them unique? How does the order know to update my computer when my package is ready and not someone elses?

That’s where gnats come in. No, not gnats, NAT. NAT stands for Network Address Translation. IP addresses in a private network can’t be transmitted over internet. So, your company has a public IP address. The public IP address, like the address on the outside of your apartment building is what the public routes to. Once the mail gets to yoru building, then it is routed via your internal network. In your case, apartment numbers. In the case of the internet, your private IP address.

Let’s suppose you and your office mate are both logging into (During a break, of course.) You both have internal IP addresses. You both make a connection to through your company’s internet connection. Amazon’s servers see two connection. They both are coming from your company’s public IP address. Amazon’s servers respond and the packets go back to your company router.

So, how does your company router now to send you the listing for trampolines and sent your coworker the listing for “Authentic snow sandles”?

Your company’s router is smart. When it translated the private IP address for your computer to the company public IP, before routing it on to, it put the information for your computer’s address into a specific port. It’s part of the TCP packets that get sent. The router assigned your office mate to a different port. And it keeps track of which computer is assigned to which port.

The interesting thing about NATing is that you don’t have to worry about it. The network router when properly set up does it all on its own. You don’t even know that your traffic is being NATted.

There’s one more address your computer has that has nothing to do with the internet address. Tomorrow we’ll talk a little about MAC addressing.

And it has nothing to do with McDonalds.

Stay safe

Rodney M Bliss is an author, columnist and IT Consultant. His blog updates every weekday. He lives in Pleasant Grove, UT with his lovely wife, thirteen children and grandchildren.

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(c) 2020 Rodney M Bliss, all rights reserved

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