You know how to find your IP address, right? You can just run ifconfig and it will tell you.  Now the big question: How did your computer get that IP address? It didn’t buy one at the IP store or pull one out of the closet, it had to get it somehow. Well, there’s two places it
could have come from, and in this post we’ll talk about where your IP address came from.

The first choice is called a static assignment. That means your ISP picked an IP from the collection it has and assigned it to you.

Home address

It’s like the address of your house. It isn’t going to change, it’s always going to be that address. Unless you move your house completely, then your house address will change. If you move ISPs, then your IP address would change.

This used to be the way IP addresses were assigned to everyone. Remember how I said we’re running out of IP addresses? That doesn’t work as well any more. Now we have a dynamic pool.

Dead Pool

Not Dead Pool, but a dynamic pool of IP addresses. When you tell your ISP ‘Hey, I’m going online’ it takes an IP address from its pool and assigns it to you. When you turn off your computer or your router, you give that IP address back to the pool.

Swimming Pool

Well, not into the swimming pool but rather the collection of IP addresses.

This is called DHCP, or Dynamic Host Configuration Protocol. That’s quite the mouthful, so we’ll stick with DHCP.

DHCP usually comes with a timer. That means you have to renew your IP address periodically. Normally it just means you say ‘yes, I’m here, I’m still using it, go away’ but depending on how the configuration works, you might get a new IP address. In other words, your IP addresses aren’t set in stone like your house address, but are likely to change. Like if you had a houseboat and moved it often, then that house address would change.

IP addresses are formally called IPv4 addresses.  This post is to disuss the next generation of IP addresses, called IPv6 addresses.

This is kind of like Star Trek: The Next Generation, but without Data and Picard and the whole crew.

 Ok, it’s nothing like them.

In this post I said that there are 2³² IP addresses, or 4,294,967,296 IP addresses.  Some of these are reserved and shouldn’t be used on the Internet, but that’s still a lot of IP addresses, right?  Enough to almost give every person in the world their very own IP address.  That leaves out companies having a whole bunch of computers on the Internet, or Universities with their computers, or even light bulbs with their IP addresses.  We’re running out of IP addresses!

The smart people that work on engineering the Internet, known as the IETF decided to solve this problem.  On a side note, they really could have come up with a better name.  Maybe Super Hardworking Internet Engineers Leading Development.  That sounds awesome, right?  They’d have to come up with their own Nick Fury though.

Nick Fury

Back to the problem.   2³² IP addresses aren’t enough.  The geniuses at the IETF decided not to just double that number, but to square it, and then square that number.  That gives us 2¹²⁸ IP addresses, or 340,282,366,920,938,463,463,374,607,431,768,211,456 IP addresses.

Have fun reading that number out loud!

They also changed the notation for the IP addresses.  They don’t look like IPv4 addresses, they use : instead of . to separate numbers.  They also use base 16 instead of base 10 for the addresses.  Bet you didn’t think there would be math in this blog, did ya.

Here’s an example of an IPv6 address:

2601:547:902:cba7:4b8:ff43:f419:cd01

That’s a mouthful to remember.  I have an easier time remembering 192.168.0.1.

The IPv6 protocol that the SHIELD, okay, IETF engineers designed does a lot more than just creating the IP addresses, but that’s all we’re concerned with for right now.

Now remember that ifconfig output?

eth0      Link encap:Ethernet  HWaddr 00:1A:2B:3C:00:00
          inet addr:192.168.0.10  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fd91:ae3b:d5e8:aa85:0:0:0:0 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:41620 errors:0 dropped:0 overruns:0 frame:0
          TX packets:40231 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:21601203 (20.6 MiB)  TX bytes:6145876 (5.8 MiB)
          Interrupt:21 Base address:0xe000

The third line is the IPv6 address of the system.  A system can have both kinds on one interface.  Neat, huh.

We’ve talked about IPv6, we’ve talked about broadcast and other parts of the ifconfig output, but there’s more ifconfig output to look at.   The rest of the story, as it were.  So we have this output:

eth0      Link encap:Ethernet  HWaddr 00:1A:2B:3C:00:00
          inet addr:192.168.0.10  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fd91:ae3b:d5e8:aa85:0:0:0:0 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:41620 errors:0 dropped:0 overruns:0 frame:0
          TX packets:40231 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:21601203 (20.6 MiB)  TX bytes:6145876 (5.8 MiB)
          Interrupt:21 Base address:0xe000

eth0 is the interface for your computer to the internet.  That thing that all the traffic goes through.  That means that there’s Internet traffic going through that device.   ifconfig shows us more information than just the IP address.

Let’s skip to these two lines:

          RX packets:41620 errors:0 dropped:0 overruns:0 frame:0
          TX packets:40231 errors:0 dropped:0 overruns:0 carrier:0

The Internet breaks all traffic up into packages called packets.  Think of it like a really really long text.   Every 160 characters in your text, your phone will send along the message and wait for the next bit.  (That 160 characters depends on your phone.  It could be more, it could be less.)  Your computer does the same thing.  It takes your messages and breaks them into chunks and sends it through the Interface.  It can also receive things through the interface.

The RX packets packages string in the ifconfig output is the number of packets your computer received and the TX packets is the number of packets your computer transmits.

Now these lines:

RX bytes:21601203 (20.6 MiB)  TX bytes:6145876 (5.8 MiB)

Are the total amount of traffic that went through all those pipes.  We received 20.6 megabytes of data and we sent out 5.8 megabytes of data.  We were busy surfing the web and pulled down the fun stuff.

The last bit of the output that we’re going to talk about is this line:

collisions:0 

A collision occurs when two devices on the same network try to transmit at the same time.  ifconfig keeps track of this because it usually means that there’s something wrong with the network and it should be looked at.

Now we’ve discussed all the important parts of the network interface!  If you want more information, read this site.

If the Avengers: Age of Ultron taught us anything, its that sometimes ideas fail spectacularly. Case in point, Ultron wanted to destroy the world to save that. That was a big mistake by Tony Stark, and not just a big mistake, a  BIG BIG mistake. On the Internet, sometimes things fail as well, just not as spectacularly as Ultron, but still, a failure. There’s a protocol to tell us that something went wrong, it’s called ICMP. It’s not nearly as dangerous as Ultron trying to make the world explode, and that’s a good thing.

Ultron

In this post I said there were 65535 ports on a system. What happens if you try to access port 99999? That port doesn’t exist!

ERROR ERROR

The good news is that your computer won’t blow up. That would be a good plot for a movie, don’t you think? Send a connection to a system on a particular port and watch the computer go BOOM. (Hollywood, if you use that, I thought of it first.)

The other good news is that you won’t even notice it when it happen, unless you’re sitting there watching every little bit of traffic coming to your computer (if so you need a new hobby). Your computer silently handles the problem by sending an ICMP packet saying ‘Nope, that’s not here.’

Your computer can also send ICMP packets out. The program ping uses what’s called an ICMP ECHO_REQUEST packet to see if another computer is alive and to find out how long it takes to get there. It runs like:

Alpha:~ computerlamp$ ping www.google.com
PING www.google.com (173.194.205.103): 56 data bytes
64 bytes from 173.194.205.103: icmp_seq=0 ttl=45 time=27.505 ms
64 bytes from 173.194.205.103: icmp_seq=1 ttl=45 time=24.952 ms
64 bytes from 173.194.205.103: icmp_seq=2 ttl=45 time=23.545 ms
64 bytes from 173.194.205.103: icmp_seq=3 ttl=45 time=24.521 ms
64 bytes from 173.194.205.103: icmp_seq=4 ttl=45 time=23.499 ms
64 bytes from 173.194.205.103: icmp_seq=5 ttl=45 time=27.609 ms

It keeps going and going and going… until you use Control-C to stop it.

When I do hit control-C, it tells me some statistics about the traffic:

--- www.google.com ping statistics ---
6 packets transmitted, 6 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 23.499/25.272/27.609/1.695 ms

Or, I can use the -c flag for ping. That way it will only do the number of pings I tell it to:

Alpha:~ computerlamp$ ping www.google.com
PING www.google.com (173.194.205.103): 56 data bytes
64 bytes from 173.194.205.103: icmp_seq=0 ttl=45 time=24.166 ms
64 bytes from 173.194.205.103: icmp_seq=1 ttl=45 time=23.373 ms
64 bytes from 173.194.205.103: icmp_seq=2 ttl=45 time=23.462 ms
64 bytes from 173.194.205.103: icmp_seq=3 ttl=45 time=23.437 ms
64 bytes from 173.194.205.103: icmp_seq=4 ttl=45 time=23.783 ms

--- www.google.com ping statistics ---
5 packets transmitted, 5 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 23.373/23.644/24.166/0.297 ms

I talked about TCP and UDP in this post. Now we’re going to talk about how TCP really works, how it keeps that connection going. It’s how mail works, it’s how a lot of web traffic works, so how does that pipe keep going?

A pipe

Well, it isn’t a physical pipe like that picture. It isn’t a physical connection at all and it also isn’t a psychic connection, instead, it’s a virtual connection.

It starts by the two systems, let’s call them Captain America and Black Panther, negotiating a connection.  The connection is called a handshake.

A handshake

Actually it’s called a three-way handshake and it works like this:

Captain America tells Black Panther, “Hey, I want to talk to you.”. He does this by sending a packet called a SYN packet.

Black Panther then tells Captain America, “Gotcha! Good idea, Let’s talk.” He does this by sending back a packet called an ACK. The three-way is because Captain America acknowledges Black Panther’s agreement to talk by sending back his own ACK.

The TCP connection uses these three packets to set the connection up, SYN, ACK and ACK.

So they’re talking away, but like in all conversations, sometimes there’s a lull where neither side says anything. Maybe they’re each getting a nice drink, maybe they’re each writing a note to themselves to make fun of Iron Man, maybe they’re just taking a nap. The connection sends back and forth KEEP ALIVE packets to say ‘Hey, you still there?’ and the other side acknowledges ‘Yup, still here. Taking a nap, be back to talking with you in a minute.’

All conversations eventually end, or at least take enough of a break that you hang up the phone and do something else for a while. When this happens, the TCP connection is shutdown, like when you hang up your phone. Your phone tells the other guy ‘yup, I’m done’ and shuts down the communication. Similarly, the one end of the TCP connection will tell the other ‘I’m outta here’, which ends the connection.