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.

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.

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!