How do you know where to start subnetting?

gbdavidx

Trying to subnet 10.39.144.178 with a subnett of 255.255.255.240. I get stuck all the way until i have to figure out where to start the network. I understand its a class A network, but since its w/ a class C subnet wouldn't the class begin at 10.39.144.0? and start my increment from there?

TechGuru80

255.255.255.240 means this is a CLASSLESS network. A classful class A address would be 255.0.0.0.

Ok so ignore any type of class.

This means our increments go into the last octet because it is not "full".

How many bits is 240? (128 + 64 + 32 + 16 = 240). Ok so we have 4 bits for the network, meaning we can do 2^4 = 16 networks with 16 hosts each.

10.39.144.0-15
16-31
32-63
64-91
92-127
128-159
160-175
10.39.144.176-191*******************
192-

So you were on the right track but immediately ignore the classes if the subnet is not correct for that class.

gbdavidx

I also get confused with class "a" networks... if you have a 10.17.164.0 /19 I understand its the same type a network, I was able to subnet the last example fine but somehow I am still missing something. Wouldn't this be a 10.17.0.0 since it is borrowing from the 3rd subnet mask range?

this one i messed up because i used the wrong increment... oops

EdTheLad

10.17.164.0/19; /19 = 255.255.224.0

256 - 224 = 32; subnets in 3rd octet are increments of 32

32+32+32 .... 5 x 32 = 160; 164 lies between 160 and 192, therefore 164 is in the 160 subnet, following subnet is 192.

Subnet 160.0
First host 160.1
Last host 191.254
Broadcast address 191.255

Then throw the 10.17.x.x on the beginning.