Cutting Down The Time Spent On Subnetting Questions Figuring out the number networks increment by (Magic number). Take 256 minus the subnet mask= magic number. Example: 128.0.0.0 256 minus 128= 128 255.192.0.0 256 minus 192= 64 255.255.224.0 256 minus 224= 32 255.255.255.248 256 minus 248= 8 Figuring out which network a particular host belongs to. Take 172.26.180.185 255.255.248.0 for example. 172.26 we don't worry about, and same with the .185 Take 256 minus the 248 mask = 8 (increments of 8 to be exact) Magic number!!! Take 180 and divide by 8 = 22.5 All you care about is the whole number here. drop the .5 and multiply 8 (increments) X 22 (your whole number) = 176 176 is your network number. 172.26.176.0. Take 172.24.65.168 255.255.255.128 for example. 172.24.65 we don’t worry about here Take 256 minus the 128 mask=128 (increments of 128 to be exact) Magic number!!! Take 168 and divide by 128=1.3125 all you care about is the whole number here. Drop the .3125 and multiply 128 (increments) X 1 (your whole number) = 128 128 is your network number. 172.24.65.128 What if they give you a slash notation instead of the whole subnet mask. Take 191.12.180.12 /28 To figure out the subnet mask we need to figure out what a /28 is Take /28 were borrowing 4 bits X.X.X.11110000 128+64+32+16=240 (subnet mask) 1st bit borrowed= 128 nd 2 bit borrowed= 192 rd 3 bit borrowed= 224 th 4 bit borrowed= 240 th 5 bit borrowed= 248 th 6 bit borrowed= 252 th 7 bit borrowed= 254 th 8 bit borrowed= 255 So we know the IP address is 191.12.180.12 The subnet mask is 255.255.255.240 From here follow the above examples 191.12.180. we don’t worry about Take 256 minus 240 mask= 16 (increments of 16 to be exact) Magic number!!! Take 12 and divide by 16= .75 all you care about is the whole number here. Drop the .75 and multiply 16 (increments) X 0 (your whole number) = 0 0 is your network number. 191.12.180.0. Take 172.21.98.116 /23 To figure out the subnet mask we need to figure out what a /23 is Take /23 were borrowing 7 bits X.X.11111110.0 128+64+32+16+8+4+2=254 (subnet mask 1st bit borrowed= 128 nd 2 bit borrowed= 192 rd 3 bit borrowed= 224 th 4 bit borrowed= 240 th 5 bit borrowed= 248 th 6 bit borrowed= 252 th 7 bit borrowed= 254 th 8 bit borrowed= 255 So we know the ip address is 172.21.98.116 The subnet mask is 255.255.254.0 From here follow the above examples 172.21. we don’t worry about, and the same with 116 Take 256 minus 254 mask= 2 (increments of 2 to be exact) Magic number!!! Take 98 and divide by 2= 49 all you care about is the whole number here. multiply 2 (increments) X 49 (your whole number) = 98 98 is your network number. 172.21.98.0. Figuring out how many subnets and hosts per subnet can you get from a network. Take 192.168.95.0/27 for example We know a 192.X.X.X address is a class C address. Or in this case 24 bits. (classful). Class A 0-126, or 8 bits example 124.X.X.X Class B 128-191, or 16 bits example 128.X.X.X Class C 192-223, or 24 bits example 192.X.X.X To figure subnets So take the slash notation were given of /27 minus it from the number of bits in our classful IP addresss 24=3 (remaining bits) this gives us 2^3=8 (Number of subnets). To figure out hosts So take the slash notation were given of /27 minus it from the number of bits in an Ipv4 address 32=5 (remaining bits) This gives us 2^5=32 minus 2=30(number of usable hosts). For the network 192.168.5.0 /27 you have 8 subnets, and 30 hosts Take 128.23.14.0/28 for example We know a 128.X.X.X address is a class B address. Or in this case 16 bits. (classful). To figure subnets So take the slash notation were given of /28 minus it from the number of bits in our classful IP addresss 16=12 (remaining bits) this gives us 2^12=4096 (Number of subnets). To figure out hosts So take the slash notation were given of /28 minus it from the number of bits in an Ipv4 address 32=4 (remaining bits) This gives us 2^4=16 minus 2=14(number of usable hosts). For the network 128.23.14.0 /28 you have 4096 subnets, and 14 hosts. Don’t let exponents fool you (2^x). It’s not as hard as they seem. Take 2^10 for example You will always start with the number 2 double the number until you reach the second number 2 (1st time) 4 (2nd time) 8 (3rd time) th 16 (4 time) 32 (5th time) 64 (6th time) 128 (7th time) 256 (8th time) 512 (9th time) 1024 (10th time) So we took the number 2 and doubled it 10 times for the number 1024. Take 2^6 for example 2 (1st time) 4 (2nd time) 8 (3rd time) th 16 (4 time) 32 (5th time) 64 (6th time) 2^6=64 So we took the 2 and doubled 6 times for the number 64 The main key to subnetting is practice, practice, practice. A great web site to practice is http://www.subnettingquestions.com. If you have any questions in reguards to subnetting feel free to email me at shayne.cda1@yahoo.com