Subnetting Cisco CCNA -Part 4 The Magic Number

okay here we go one more time with the subnetting magic number situation subnetting magic number trick right okay so they give you an IP address like 192 168 1 . 153 and they say this time it's slash 27 okay so first of all you need to know well what is / 27 mean well first of all we have well we have to figure out what is the magic number so slash 27 so it's very easy I just will go like this 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 and that is let's see here put a dot there but I doubt there and a dot there that's 24 ones and then 25 26 27 1 2 3 4 5 and then there's my zeros right let's make it smaller okay so there's my subnet mask all right and if I was to write it in decimal notation it would be 255.255.255.0 4 right well how do I know that once again binary this is in the 128 place this is in the 64's place and this is in the 32s place why 2 to the 7th 2 & 6 2 to the 5th 2 to the 0 1 2 3 4 5 6 7 so the 128 plus 64 plus 32 is 2 24 right what's my magic number here it is my magic number is the last borrowed bit in this case it's this and we know right off the bat that what place value it is in is the 32s place so the magic number equals 32 all right so now I can find out I can answer any question they give me related to this IP address and number for instance they could say to me you know is it a broadcast number is it a network number what now first of all let's find out what network is it in okay so since it's a since the magic number is 32 in the last borrowed bit is a 1 in the 32 s place I know that the networks go like this 0 32 64 plus 32 is 96 plus 32 is 128 right plus 32 is 160 okay and right off the bat I've passed my IP address so I know I have the answer all right the answer is 153 is in between the 128 network and the 160 Network so I know right off the bat that the network is 192.168.1.1 28 so the 128 network the broadcast address in that network is 159 so if they ask me what's the network for this number right here right 150 192 168 1 dot 153 with the slash 27 I can answer them very quickly that the network is the 128 network the broadcast address is 159 and that yes it is a useable host address in that network okay I can also tell them that it's the 1 2 3 4 v network right I can tell them that there are the magic numbers 32 so I can tell them that there are I can tell them that there are 32 hosts in the network right 32 hosts right we've borrowed three ones here three subnet bits so I know that they're the the networks there are 2 times 2 times 2 2 times 2 is 4 times 2 is 8 so I know that there are 8 networks right 8 networks there are eight networks that were created I know the networks go up by 32 that's the most important part 0:32 6496 128 160 the next one is 192 alright the network next one is 224 and that's it so I said that there was let's see I said there was 8 networks and just count them up right 1 2 3 4 5 6 7 8 those are the 8 networks so I can learn a lot from the magic number theory in answering the subnets right here's another question they probably will give you on the CCNA I'll ask you a question like this you know Bobby has a you know they'll say the admin network has 44 users right then I'll say the teacher network has you know 123 users right you know what should the what should be the subnet mask in what you know how would how would they sup which would be the proper slash subnet mask notation would it be / 24/25 / 26 right something like that or slash 27 right well slash 24 is classful and that means there's 255 hosts slash 25 we've borrowed one bit right and it's in the 128 spot so we know there's 128 hosts right that's going to be enough hosts the teacher network let me write that out that's bothering me has 123 years okay / 25 has 128 hosts and the teacher Network has 123 and this one has 44 so that's going to be enough host to cover this network now remember 128 host and / 25 because / 25 the last octet is this okay one more zero the last octet looks like this dot 10.0.0.0 the one is in the 128 place right so that's how I know that there is the magic number is one is 128 the magic number is 128 and that means there's 128 hosts that's enough host for this network you can't use the first number and you can't use the last number so there's actually only 126 useable hosts okay that's an important distinction on the test right it's an important distinction let me make that smaller there we go usable 126 useable holes so anyway this is going to be the correct answer slash 26 is the magic number is 64 so that's 64 hosts and with this borrowed three bits 25 26 27 the magic number is 32 so it's 32 hosts 64 hosts right so if the admin network has 44 users and the teacher Network has 123 users we're going to want to use a slash 25 also yeah in the / 25 Network that should be good yeah that worked okay next movie you

Channel: danscourses

Views: 241,634

Keywords: cisco, ccna, subnetting, made, easy

Id: RGBA83J60H0

Length: 8min 38sec (518 seconds)

Published: Mon Sep 21 2009