when you start working with IP version 4 one of the things you'll hear about are classes of IP addresses and in casual conversation we might even refer to an address as a Class A address a Class B address or a Class C address this was a method of describing an IP address as it related to its subnet mask anything that is a class A address has eight bits assigned for the network address and 24 bits that are associated with the host address this means that the subnet mask for a class A address would be 255.0.0.0 a Class B address default subnet mask is 255.255.0.0 and the default subnet mask for a class C address is 255.255.255.0 this style of class-based subnetting is one that we have not used since 1993 but we still refer to some of these addresses in a very colloquial sense and although you may hear someone refer to an address as a Class A address or a Class C address in reality we no longer use a class-based form of subnet masking so if we don't use classes why are we doing a video on class-based networking that's because these classes do act as a starting point when we begin any type of subnetting on our networks if you're going to subnet a network it's important to know where to begin the subnetting process and this class-based system is the starting point these Class A Class B and Class C addresses have an obvious demarcation between the network part of the address and the host part of the address with class A that line is drawn after the first eight bits in a class B address that line is after the first 16 bits and in a class C address that line is after the first 24 bits here's a table that defines the different subnet classes for class A Class B Class C class D and the reserved Class E and you can see if you look on the right side the default subnet masks are the ones we just saw where Class A has a default mask of 255.0.0.0 Class B is 255.255.0.0 and Class C is 255.255.255.0 the way that you can look at an IP address and tell immediately what class that IP address is associated with is the very first octet of the address address if the decimal octad is between 0 and 127 as the first number of that IP address it's a Class A address if the first number of the IP address is between 128 and 191 that's a Class B address and if the first number is between 192 and 223 that is a Class C address if you were to look at that address in binary if the first bit was Zero it's a Class A address if the first bits are one zero it's a Class B address and if the first three bits are one one0 it's a Class C address class D addresses are used for multicast communication and therefore they are not assigned to an individual device but there is a range for class D addresses and that is between 224 and 239 or the first four bits of a Class D address are 11 one0 and there is a range of IP addresses that are not used for any purpose this is a reserved range we refer to this as the class E range between two4 4 and 255 or the first four bits of 1111 So based on that chart and those ranges of IP addresses let's see what class a particular IP address would be part of let's start with the IP address of 17. 22907 to be able to determine the class we need to look at the first octet which in this case is the number 17 and if you refer back to that chart the number 17 is in a class A address let's do another one IP address is 22010 7740 if we look at that first octet it is 220 and 220 is in the cclass the next IP address is 16524 1 we need to look at that first octet in this case that's 165 and 165 is in the range of a Class B address next on our list is 128. 19010 do2 that first number is 128 which means that is also also a Class B address the IP address of 19177 24250 has the first octet as 191 and if we refer to our chart 191 is in the range of a Class B address and lastly 19 19211 12.5 we're looking at that first octet of 192 which falls in the range of a Class C address when you start calculating an IP subnet there are four different values that are very important to know the first is the IP address associated with the network address this is the one that defines all of the other devices on this particular subnet to calculate a network address you set all of the host bits of that address to zero and then determine what the decimal representation of that might be the second important piece of information you need for an IP subnet is the first usable host address on that subnet this is always going to be one number higher than this previously defined Network address you also need to know the network broadcast address this is the last IP address of this particular Network address to be able to calculate the network broadcast address you would set all of your host bits to one which is the same as 255 decimal and perform the conversion between the binary and the decimal and just as you need to know the first usable host address on the network you also need to know the last usable host address and to calculate that you look at the network broadcast address and it's one number fewer than the broadcast address so let's calculate those four important subnet values and let's start with the IP address of 10742 22.11 if we look at our chart we know that this starts with the number 10 therefore this must be a Class A address and the default subnet mask for a class A address is 255.0.0.0 since we're using this class a subnet mask we know that everything that's on the left side of our bar or this first octet is going to be our Network address and everything on the right side will be associated with the host ID so let's add those to our chart we have a network column and a host column obviously the tin dot is going to be in the network column that's what's on the left side of that bar and then everything on the right side is going to be the host address we first need to calculate the network address and to do that we set all the host bits to zero this means that everything in that second column will be zero and the network address therefore will be 10.0.0.0 the first host address is one more than the network address so our first host address would be 10.0.0 one to calculate the broadcast address for this subnet we set all the host bits to one which means that the broadcast address is 10255 255.255 and as you recall to calc calculate the last usable address on this subnet we need to subtract one from the broadcast address so the last usable address on this network is 10. 17216 188-200 if we look at that first octet we can see that that is a Class B address which means the default subnet mask for this network is 255.255.0.0 this means that we would draw our line right in the middle between the first 16 bits and the last 16 bits so everything on the left side is associated with the network address and everything on the right side is associated with the host address now let's perform those EX exact same calculations we want to determine what the network address is so we will set all host bits to zero this means that the network address for this particular subnet is 172.16.0.0 to be able to calculate the first host address we need to add one to that value or 172.16.0.0 to calculate the broadcast address we set all host bits to one which means the broadcast address for the subnet is 17216 255.255 5 and to define the last usable host address on this network we subtract one from the broadcast address which means the last usable address is 17216 255.255 let's see if we figured out this process and step through another IP address in this case 192168491 to that fits into the range of a Class C address which means the default subnet mask is 255.255.255.0 if we were to draw our line the network side of the address is the first three octets and the host side is the last octet let's put that into our chart so our first three octets 192.168.0 is the network side and77 is the host side to be able to calculate the network address we set all host bits to zero so the network address for this particular subnet is 192.168 . 4.0 if we add one to that address we get our first host address of 192.168.1 to be able to calculate the broadcast address we set all host bits to one which means the broadcast address is 192.168 4255 and if we subtract one from that broadcast address we get the last host address and that would be 192168491 subnetting we can still use the same process when we look at anything that may be class or non-class based and as we go through our additional subnetting videos you'll notice that the same process is used over and over again regardless of the IP address and the subnet mask