in a previous video we talked about class-based subnetting and how IP addresses could be assigned a Class A Class B or Class C address but since 1993 we've not used class-based subnetting instead we use classless subnetting sometimes you'll see this referred to as classless in domain routing or CER this classless addressing means that we can assign a subnet mass that doesn't fall into those same class A a Class B or Class C arrangements and instead of referring to the subnet mask in its decimal form we can simply refer to it as the number of bits that number of bits is referred to as a cider block notation for example a decimal subnet mask of 255.0.0.0 is the same as a sl8 cider block notation because it has eight individual bits as part of the subnet mask so it would be very common to see an IP address written as 192.168.1 44/24 everything with the sl24 is referring to the number of bits in a subnet mask and a sl24 mask would be the same as 255.255.255.0 when you're configuring a device you're often giving it an IP address a subnet mask a default gateway and very often a series of DNS servers most of the time an operating system is expecting you to put the subnet mask in as a decimal mask so for example in Windows you may put the IP address and then you might add in the decimal version of the subnet mask but if you're using a router or a switch it may be expecting the CER block notation for those masks so instead of using 255.0.0.0 you would simply input sl8 make sure you check the documentation of the device you're using to see what notation it perer refers for the subnet mask as you've probably seen in our previous examples of subnet masks The Mask consists of a contiguous series of ones followed by zeros so the ones would be on the left the zeros would be on the right for example here is a subnet mask you can see the contiguous set of ones on the left and the zeros that are on the right in this example you can see three separate octets that are set to one so there are 24 ones in this particular mask and there are eight zeros at the end if we were to write this out in decimal it would be 255.255.255.0 and if you were to write this out in cider block notation you would count how many ones are associated with the subnet mask in this case there are 24 so the cider block notation would be sl24 the subnet mask is obviously used to separate the network part of the address from the host part of the address so for this particular subnet mask the network address is 24 bits in length and the host address is 8 Bits in length let's convert a binary subnet mask to one that is the cider block notation for a subnet mask and we'll take this subnet mask that we have on the screen which has all ones in the first two octets and all zeros in the last two octets if we were to count the number of ones in this subnet mask we have eight in the first octet eight in the second octet none in the third and none in the fourth that means the total number of ones in the subnet mask would be 16 so the cider block notation would be sl16 this means that the first 16 bits of the subnet mask are associated with the network and the last 16 bits are associated with the host ID let's do another one in this example the first three octets are all ones but the last octet has two ones and six zeros so let's count the number of ones in the subnet mask there are eight in the first octet eight in the second octet eight in the third octet and two that are in the fourth octet if we count all of those together we have 26 so the cider block notation of this subnet mask would be sl26 the sl26 means that the first 26 bits of this address are the network part of the address and the last six bits are reserved for the host ID and here's one more in this subnet mask we have the first octet of all ones the second octet has four ones and four zeros and then the third and fourth octets are all zero since we are counting number of ones we have eight in the first octet and four in the second octet and if you count those together the subnet mask Insider notation would be a sl12 that means that the first 12 bits are for the network ID and the last 20 bits would be for the host ID we've looked at the subnet mask in binary and we've calculated the cider block notation but we often refer to the subnet mask as a decimal value if we were to convert these binary subnet masks into a decimal value there are only a certain number of these that you would run into for example all zeros would obviously be a decimal zero but anything that starts with a binary of one and the rest zeros would be a decimal of 128 if there are two ones it's 192 three would be 224 and so on this means that you could create a chart that shows all eight of the bits within a single octet and what the decimal equivalent would be we can now use that chart to calculate the cider block notation and what the decimal representation of that subnet mask would be so let's look at a subnet mask of all ones in the first octet there are four ones in the second octet and the rest of this would be all zero if you refer to our chart you can see that all ones is 255 four 1es and four zeros would be 240 and then of course all zeros would be zero and if we were to count the number of ones we know that correlates back to a sl12 so a /12 cider notation is the same thing as 25524 z.0 in decimal notation and of course since there are 12 ones the network section is 12 bits in length which leaves the remaining 20 bits available for the host ID let's look at a subnet mask we've not seen yet this one is all ones in the first octet all ones in the second octet there are three ones in the third octet and of course the rest of this is zeros if we look at our chart we can see that of course all ones is 255 and in the octet that has three of these ones we refer to the chart that is the same as 224 in decimal and obviously all zeros in binary is the same as zero decimal if we then count the number of ones in all of these octets they all add up to 19 so your cider block notation would be sl19 which corresponds back to the decimal representation of 255 25.22 4.0 this of course means that the network part of this address is 19 bits in length and the host section is 13 bits in length of course we can do this in Reverse as well let's let's say that we have a subnet Mass that's been given to us as a sl26 in cider block notation if we were to write this out in binary that means that we would have all ones in the first three octets and two ones in the last octet and if we were to refer back to our chart all ones would obviously be 255 and the last octet that has two ones correlates back to a 192 in decimal so a sl26 where your network ID is 26 bits in length and the host ID is six bits in length is the same as a sl26 Insider block notation or 255.255.255.192 in decimal let's do another one where we've been given a sl20 in cider block notation that means there are 21s in this particular subnet mask so your first two octets are all one and the third octet has four ones and four zeros and of course the last octet will be all zero if we refer to our chart we know that the decimal version of that binary subnet mask will be