Category Archives: Networking

CIDR Addressing: What is it?

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A collection of IP addresses is a network.  We want the addresses to be contiguous, that means they should be right next to each other.  We want 127.0.0.1, 127.0.0.2, 127.0.0.3, not the addresses 127.0.0.1, 192.168.5.4 and 8.8.8.8.  Those first three are next to each other, the next three are all over the Internet.  Instead of listing everything in a network, CIDR addressing lets us use one thing to mean the whole network. We could list everything out.  That would take too long.  I want a shorthand.

An IP address is also a 32 bit number.  Instead of writing 192.168.0.5,  I could do:

1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1

That’s quite a mouthful, isn’t it? All 32 1’s and 0’s.  Imagine having to type that every time.

Back to CIDR addressing and networks.  To create a network, we say, ‘okay, the first 24 of these 32  1’s and 0’s cannot change!’.  Like on the Millennium Falcon the hyperdrive goes wonky and 24 of the 32 switches Han and Chewie need to flip won’t flip.  They’re stuck.  But they can flip any of the other 8.  Of course, that’s when the Empire shows up and tries to shoot them out of the sky, but that’s not the point of CIDR addressing and networks.

Back to CIDR addressing.  We fix the first 24 bits.  That looks like:

1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0

And now we let the last 8 bits be aaaaanything they want.  They can be 1 0 0 1 1 0 0 1, 1 1 1 1 1 1 1 1, or any other combination you can dream up.  Each one of those combinations makes a number.  And (insert math magic here) an 8 bit number is anything between 0 and 255.

Moving away from the 1’s and 0’s for a moment, what we’ve done is said ‘ok, the first part of my network is 192.168.0’.  The ‘second part of my network is any number between 0 and 255’.

In CIDR addressing terms, we’ve made 192.168.0/24.  That 24 is from those 24 1’s and 0’s we fixed.

You can fix any number of bits between 0 and 32.  If you fix 0 bits, then you’re listing every single IP address in the Internet.  If you fix all the bits, well, your network is just a single IP address.

Netmask

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The last post talked about how to find your gateway, also known as your first hop.  In the output to netstat there was a weird thing called a Genmask.  We’re going to talk about that and something called a netmask this time.

It isn’t the same thing as the mask Batman wears.  It isn’t even the same thing as the helmet Iron Man wears.  It’d be cool if it was, then we could have a helmet over the computer and wouldn’t that just be too cool for words?  But it isn’t, it’s just a fancy way of saying something else.

So the first thing, your computer is part of a local network.  It might not seem that way because you’re using a cable provider or DSL or wireless at a library or however you’re using the Internet, but there’s a local network your computer is part of.  We’ll pretend it’s 256 computers.  It makes things easier to begin with.

The first part of your IP address is the network that it is a part of.  So if your IP address is 192.168.0.25 then the network part of that IP address is 192.168.0.  (Why is that true?  Remember how I said that every part of an IP address is a number between 0 and 255? So that last dotted quad can be any number between  0 and 255 and there’s 256 numbers right there.)

Back to talking about networks!  We now know what your network is, that’s a start.  We also know that there’s 255 possible other IP addresses in that network.

The netmask (or subnet mask or network mask your choice!) is another way of saying this.  The netmask for your network is 255.255.255.0.  That means that the first three quads are fixed and the last is allowed to roam free!  Free!  Free as a bird!

Wait, I digress.  It also means that those are the computers you don’t need routing for because you’re all together.  There’s all different sizes of networks, next time we’ll talk about some more and how we write them in shorthand notation.

Wait, I almost left off genmask!

Alpha:~ computerlamp$ netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.0.2.0       0.0.0.0         UG        0 0          0 eth0

If 255.255.255.0 means that that last quad can be anything, then 0.0.0.0 means that, well, anything goes!  You can put any IP address in those spots that you want  In other words, this Gateway (192.0.2.0) is how you get anywhere from your computer, no matter what.

That First Hop

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In the last post we talked about how your data moves around the Internet, which is neat and all, but how does your computer know how to take that first hop?  Somehow your data has to get onto the route to get to www.google.com.  When you get into a car to take a ride to the comic book store, or the gaming store, or the computer store, you open the door to your home and step out through that.  That’s what your computer does, it takes the gateway to the Internet to get anywhere on the Internet.

Like in Star Trek, when they step onto the transporter to go somewhere, that’s their first hop.  Or in Portal, when you jump through the portal to go anywhere, that’s your first hop.

But what’s the computer’s first hop? If you’re reading this blog from your home Linux® system, you had to get here somehow.

And of course, there’s a command for that!  It’s called netstat.

Alpha:~ computerlamp$ netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.0.2.0       0.0.0.0         UG        0 0          0 eth0

Now the next question:  What in the world does that mean?  Looks kind of like nonsense, right?  Kind of what you’d see when watching The Avengers, they’d show it on one of the screens so it would look official, even if it is just a status report.

The Iface at the end of the report is the hardware that the computer uses to connect to the Internet.  It’s usually called eth0, or ethernet 0.  If you have multiple interfaces on your computer, you might see eth1, eth2, eth3, or more.  Sometimes you’ll see lo0 as an interface.  That’s the interface that the computer uses to talk to itself on the Internet.  Isn’t that weird?  Your computer talks to itself on the network and to make that happen, it has a hardware interface to do it.

The IP address under Gateway is the IP address that your computer uses as its first hop.  We’ll talk about masks next time, for now, you can read more in the man page.  Of course there’s a man page, there’s always a man page!

 

More about Networking, aka Route my Traffic

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So we talked about IP addresses, we talked about how to turn a domain name into an IP address… so the next question is, how do we get there from here?  I mean, how does my computer access that domain that I want to see?  Well, it’s called routing or taking a route.

When I leave my house and go to the comic book store, I take a route.  I can go down back roads or main roads or even sneak through back alleys if I’m walking.  I could even take a detour through the Batcave (if I knew where it was) or sneak through Stark Enterprises (If I knew where that was!).

But I’m lazy.  Taking all those detours or back alleys or even back roads is just the long way and I really want to get there as quick as possible.  I want the latest Spiderman, please, or the latest Wonder Woman.  I don’t want to spend all my time getting there.

Well, the Internet works the same way.

When you go from your computer to www.google.com, you send your traffic on a route through the Internet.  That route takes you through various computers with a special tasks, known as routers.  You hop from one router to the next until you reach that magic destination.  Which is kind of why each step is known as a hop.  Think of it as a magical rabbit carrying your information from one computer to the next until he reaches his rabbit hole.  Or don’t, that’s rather silly.

Of course, there’s a command to see those hops.  It’s called traceroute.

You can run this from your command like:

Alpha:~ computerlamp$ traceroute www.google.com
traceroute to www.google.com (172.217.4.164), 30 hops max, 60 byte packets
 1 ip-69-163-160-1.dreamhost.com (69.163.160.1) 2.273 ms 2.246 ms 2.229 ms
 2 pdx1-cr-1.sd.dreamhost.com (66.33.200.2) 0.211 ms pdx1-cr-2.sd.dreamhost.com (66.33.200.3) 0.229 ms 0.268 ms
 3 pdx1-bdr-2.sd.dreamhost.com (66.33.200.1) 0.239 ms pdx1-bdr-1.sd.dreamhost.com (66.33.200.0) 0.234 ms 0.262 ms
 4 pdx1-bdr-1.sd.dreamhost.com (66.33.200.0) 0.248 ms google.nwax.net (198.32.195.34) 5.565 ms pdx1-bdr-1.sd.dreamhost.com (66.33.200.0) 0.215 ms
 5 google.nwax.net (198.32.195.34) 5.589 ms 6.025 ms 108.170.245.114 (108.170.245.114) 5.611 ms
 6 209.85.248.159 (209.85.248.159) 6.152 ms 108.170.245.98 (108.170.245.98) 5.733 ms 66.249.94.201 (66.249.94.201) 6.185 ms
 7 209.85.250.126 (209.85.250.126) 8.912 ms 72.14.238.38 (72.14.238.38) 9.633 ms 209.85.250.126 (209.85.250.126) 8.765 ms
 8 74.125.37.136 (74.125.37.136) 30.852 ms 30.771 ms 209.85.250.126 (209.85.250.126) 8.087 ms
 9 209.85.248.124 (209.85.248.124) 30.571 ms 74.125.37.136 (74.125.37.136) 29.833 ms 209.85.247.0 (209.85.247.0) 29.980 ms
10 108.170.247.129 (108.170.247.129) 31.130 ms 209.85.247.0 (209.85.247.0) 30.637 ms 30.647 ms
11 108.170.247.161 (108.170.247.161) 31.333 ms 31.123 ms 108.170.247.129 (108.170.247.129) 30.494 ms
12 108.170.234.215 (108.170.234.215) 30.513 ms lax28s01-in-f164.1e100.net (172.217.4.164) 30.423 ms 108.170.234.213 (108.170.234.213) 30.562 ms

That’s twelve hops to get there from here!

Or I could use what is called a Looking Glass Utility and do it on a webpage.  Global Crossing has such a Utility.  This lets you see the route from somewhere else, rather than from your computer.

traceroute doesn’t always work.  Sometimes companies block the kind of traffic that traceroute uses.  Try using it to go to www.marvel.com and see what happens!  If you

 

DNS

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In the last post we talked about how computers only understand numbers not names, in this post we’ll talk DNS. It’s how the computer knows what number goes with what name without knowing every
possible combination.

IP addresses can also change, so a computer would not only have to keep up with every possible combination but also all of the possible changes every second of every day.  Remember, there’s 4294967296 IP addresses. This translates to ‘a lot of them’. I’m not sure even a superhero could do that without help. Hrm. Maybe Batman or Iron Man could build something.

Now in the old days of the internet, this was possible. It wasn’t that big. Now… it’s that big. I mean, we’re running out of IP addresses and there’s 4294967296 of them.  Since we have no superheroes to solve this, we had to rely on the geniuses that created the whole thing to begin with. They’re the superheroes of the Internet!

Back to what DNS is. It’s a system that takes a name and returns an IP address. When you open your web browser to go to www.google.com, the first action it takes is to find that IP address using DNS. It asks the server known as a name server.   It doesn’t just ask any name server though, it asks the servers known as the roots.

The roots are a collection of name servers that only know where other name servers are.  It’s like a super search engine.  Suppose there’s a search engine that tells you if you need to go to Google or Bing or Yahoo or even Wikipedia to do your search.  You have to go to the super search engine first, then to that engine, then FINALLY to your answer.

In the case of DNS, you go first to the roots (that is, super search engine) who tells you what name servers know the answer for domains that end with com, and then that name server will tell you the name server that knows the answer for google.com.

No one name server knows all, it’s what is called a distributed system. This means if one name server is taken off line, it still works. Unlike the Death Star, that’s a single system. Blow that thing up and there goes the Empire!  Or those droids in Phantom Menace.  They blew up the one ship that controlled all the droids and WHOOPS, there goes all the droids offline!

If you want to find the IP address for www.google.com, there’s a command for that.

Alpha:~ computerlamp$ host www.google.com

Try it out, see what you get!

Networking

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Let’s change the pace a little bit, we’ve talked about using the command line to do things, now we’re going to talk about the computer network and IP addresses.  That’s also know as networking.

These days, computers do everything. Planes, trains, automobiles, your house, your phone, everything has computers. Everything is on the network too! You can even buy  light bulbs that you can talk to over a network connection. Isn’t that weird? I can’t wait for my light bulb to message me ‘I’m dying, please replace me.’ I wonder if Batman has network enabled gear yet. Can you imagine his Batmobile sending a message saying ‘My tire is low’. I wonder if it would send that to Alfred instead of Batman…

Anyway, back to the computers doing everything. One important thing to know is that they don’t understand words, they understand numbers. If you tell your phone ‘connect me to the Batmobile’ it won’t understand the word ‘Batmobile’, it has to turn it into a number it will understand. (And if your phone does connect you to the Batmobile, tell me how you did it? I want to talk to it too!)

Those numbers are called IP addresses.  An IP address (also technically called an IPv4 address) is a number between 0 and 4294967296.  Can you imagine having to remember the IP address of your computer if it was just an integer?  2130706433 is an awful number to have to memorize.  Luckily, the geniuses who designed everything came up with dotted quad notation.  It’s a bit mathy (wibble wobble effect here) but instead of memorizing 2130706433, you’d memorize 127.0.0.1.  That’s four numbers to remember with dots in between, which is why they called it dotted quad.

This website has a converter so you can see what the integer is for the IP address of your computer.

But first, you’ve gotta know the IP address of your computer before you can use that website.  There’s a command for that!

Alpha:~ computerlamp$ ip addr show

Try it out and see what you get!