welcome back in a recent video we discussed how  addresses are important devices need them so they  

know where to send their data IP addressing is  a good example of this not only does it include  

the address of the device but the address of the  network that the device is in that means with one  

simple address we can find a device on our local  network or on the other side of the world IP  

addresses come in two different flavors these are  called ipv4 and ipv6 and they look quite different  

ipv6 is newer but ipv4 is still more common so  we're going to focus our attention entirely on  

ipv4 for now firstly this is what an IP address  looks like it's four numbers separated by dots  

each of these numbers is called an octet as each  number is an 8-bit value we break it up into four  

separate octet to make it easier for us humans  oh and if you're having trouble remembering one  

octet is think of an octopus an octopus is eight  tentacles and octet has eight bits eight bits  

means that these numbers can range from 0 to 255  that means that an IP address starts at 0 0 0 0  

and ends at 255 dot 255 dot 255 dot 255 we call a  range of addresses the IP space straight away you  

can see that knowing how binary works is important  for IP addressing if you don't know a binary or if  

you need a refresher I have a video to help  I'll add it to the description IP addresses  

are actually two addresses in one the IP address  is the address of the device but it's also the  

address of the network that the device is in two  addresses in one how is this possible let's take a  

look at this sample IP address here 172 dot 16 dot  0 dot 1 the first part 1 7 2 dot 16 refers to the  

network the second part 0.1 is the host address  don't worry I'll get to explaining how we know the  

difference between the two soon all the hosts here  that start with 1:7 2.16 are in the same network  

if we add more hosts and the IP addresses start  with 1:7 2.17 then this is a different network if  

hosts in different IP networks need to communicate  then then we'll need to be a router between them

you're probably wondering is it always the first  half of the address that represents the network  

short answer no how then do we know which part  of the IP addresses the network and which part  

is the host the answer to this question strangely  has changed over time so let's start back at the  

beginning and we'll work along from there when  the Internet Protocol was first created the  

first octet always represented the network and  the following three octet were used for hosts  

the largest value that we can get out of a single  octet is 255 that means that there was only room  

for 255 different networks on the other hand three  octets meant that there could be over 16 million  

hosts IPS in each network this sounds a little  unbalanced but in the beginning the internet  

was not international and there were only a few  organizations using it but as the internet started  

to grow it became obvious that this wouldn't work  for very long 255 networks just wasn't enough so  

back in 1981 a new method was introduced this  broke the entire IP space into five classes  

that's Class A through to E Class A B and C were  used to address devices Class D is for multicast  

and we'll touch on that a little in the next video  Class E is reserved for special purposes so Class  

A B and C are the ones we need to think about  right now there are a small number of Class A  

networks but each network supports a large number  of hosts Class A works a bit like the old method  

the first octet is the network and the remaining  three octet are for hosts however the first bit of  

the network is always 0 that leaves seven bits for  us to allocate to our networks that means there  

are 128 Class A networks with more than 16 million  host IPS per Network the Class A address base then  

is 0.000 through to one 27000 there is a small  catch with this though networks starting with 0  

and 127 are reserved so the usable class a space  is really 1.000 to 120 6.000 Class B networks are  

used for a medium number of hosts the first two  octets are used for the network and the second  

two are used for host addresses the first two bits  of the network are always 1 0 which leaves 14 bits  

or 16384 possible networks each network can have  over 65,000 hosts the total Class B IP space is  

from 1 28000 to 191 to 5500 Class C are small  networks but there are a lot of them the first  

3 octets are just for the network the first three  bits are always 1 1 0 leaving 21 Network bits or  

a little over 2 million networks with only 1  octet left though we can have only 256 hosts  

ip's per Class C Network the address space ranges  from 190 to 0 0 0 2 223 255 255 0 so to summarize  

Class A uses 1 octet - 1 bit for networks Class B  uses two octets - 2 bits and finally Class C uses  

3 octet - 3 bits addresses outside of this our  classes D and E which are reserved for special  

uses if you're the kind of person that likes the  math we can show this as an equation the number of  

networks is 2 to the power of n and is the number  of network bits so Class B has 14 network bits to  

the power of 14 years 16384 the same is true for  hosts B has 16 bits four hosts two to the power  

of 16 is about 65,000 ready for some practice  take a look at these IP addresses which classes  

are they in time to look at an example if device  170 2.16 dot zero dot one wants to send a packet  

to 172 1602 it starts by looking at the first few  bits of the destination IP address as the first  

two bits are 1 0 it knows that this is a Class B  address it can then assume that the first 2 octet  

are the network and the last two are the host  it sees that the destination network is 172 dot  

16 which is the same as its own as they're in  the same network it can send traffic directly  

to the destination now what if it wants to send  a packet to 172 dot 17.0 to 1 it does the same  

thing and it determines that the destination is  on the 170 2.17 network this is different to its  

own network so it can't send traffic directly it  needs to send traffic to a router first take the  

practice session from before a step further tell  me which part of each address is the host address

as the internet started to grow we started using  up IP addresses faster and faster and we started  

running low on IP addresses again so in 1993 yet  another new method was introduced it is called  

classless inter-domain routing or cider previously  we could identify IP address class by looking at  

the first few bits knowing the class we then knew  which parts of the IP were for the network and  

which parts were for the hosts but now we throw  all that away and we introduce something new the  

subnet mask the subnet mask is also made up of  four octet s-- this lines up with the IP address  

bit for bit the bits set to one tell us which part  of the IP address is the network the zero bits are  

used for hosts it's important to notice that all  the ones go on the left and all the zeros go on  

the right we never mix up the ones and zeros in a  subnet mask take a Class A address for an example  

we know eight bits are used in the network so  the subnet mask would look like this eight one  

bits for the network the remaining 24 bits set to  zero for hosts a Class B address follows the same  

rules sixteen bits for the network set to 1 and  the host bits set to zero and finally Class C well  

you get the idea by now is this making sense  so far the subnet mask tells us which part of  

the IP is for the network and which party's for  the hosts but this hasn't really addressed our  

original problem yet has it so far we've just  seen a different way to show Class A B and C  

addresses so far we haven't seen anything on how  to save IP addresses the real power of cider is  

the ability to break a large network into small  ones we call this subnetting let's say that you  

have the classful network of 170 to 1600 this  would have the subnet mask of 255 255 0 0 this  

allows for about 65,000 hosts that's a lot of  hosts that might be fine if we have one massive  

office but what if we have several smaller offices  do we want to allocate 65,000 IPS to each office  

that seems like overkill to me so instead what we  can do is we can break the Class B Network using  

our subnet mask we may decide to use 255 255 255  dot 0 instead now our large office network has  

been broken into 256 different subnets why 256  well our subnet mask is now using an additional  

8 bits for the network the largest 8-bit value is  256 now we can allocate 256 host IPS per network  

which is far more reasonable when we have a few  small offices if we want to communicate between  

our subnets just like before we will need a router  let me take a moment to make sure that I'm clear  

on this devices in the same subnet must be in  the same IP network if not they will not be able  

to communicate likewise if devices in the same  subnet are separated by a router they will also  

not be able to communicate I'd like to challenge  you to think this through for yourself take the  

172 dot 1600 network with a subnet mask 255 255 0  0 if we broke that network up by using the subnet  

mask of 255 255 to 40.0 how many subnets do  we have how many host IPS per subnet are there  

if you're taking notes you've probably noticed  that's a pain to write down the subnet masks all  

the time to make things simple we have what's  known as cider notation take 1 7 2 dot 16 dot  

one dot 0 / 24 as an example the slash 24 is the  subnet mask written inside a notation this means  

that the first 24 bits of the subnet mask are  turned on it's that easy we will be using side  

of notation more and more from here on so try to  get more comfortable with it see if you understand  

by testing yourself with this example we have to  slash 24 networks these are joined together by a  

small slash 30 network a device in network a is  unable to communicate with a device in network  

B why are they unable to communicate classful is  an older method of networking and subnetting is  

newer so why are we even talking about classful  networking hasn't it been replaced yes it has  

been replaced but for one if you're doing an exam  you may still find exam questions that refer to  

class for addressing we also have some remnants  of classes in everyday networking think of when  

you set an IP address in Windows for example  if you configure an IP of 10.8 62 windows will  

automatically give you the subnet to 55000 it's  assuming classful networking and what we're mostly  

classless now a lot of people tend to think of  subnetting are starting with a classful network  

and breaking it up from there which is not always  strictly true sometimes we may have a few small  

networks and we decide to join them together this  is called super netting for example it may have  

one nine two one six eight zero zero slash 24  and one nine two one six eight one zero slash  

24 if we want we can join these into a single  network 192.168.0.0 slash twenty-three in my  

opinion we shouldn't really worry about class  all networks in the real world use subnetting  

all the way but what do you think tell me if  you agree or disagree in the comments I would  

love to hear your opinion join me the next video  and we'll dive deeper into IP addressing concepts