Remembering tricky Private addresses with 15, or is there a reason for 15?

Jon1992

So 15 is the number i found in 3 instances that i and anyone that needs a trick to remember certain private addresses should know. Such as Class A private address is 10.0.0.1 - 10.0.0.255 , class C is 192.168.0.1 to 192.168.255.255 , yes it doesn't use the entire 192 address block but most of us know that from dealing with Lans at home we usually use 192.168. Now Class B always confused me because its take a chunk of address's 172.16.0.1 - 172.31.255.255 unlike class A and C it doesn't just use 172.16.0.1 - 172.16.255.255 so it was hard for me to remember where it started and ended but 16 + (15) = 31, 172.16.0.1 - 172.(31).255.255 . Now on to class D, E . you could start from class C which ends at 223 , So D starts with 224, add 15 you get 239 so theirs class D 224-239, class E would start at 240 add 15 = 255. or start at 255 minus 15 = 240 . so you know E is 240-255 then 239 - 15 = 224 so theirs class D. IDK if this is a number they chose to use are maybe there is something im missing.

PCTechLinc

As far as why they chose 172.16.0.0 to 172.31.255.255, it was just chosen at random.

To explain the rest, you need to break down the first octet into binary:

Class A must start with a 0, Class B must start with a 10, Class C 110, Class D 1110, and Class E 1111.

A = 00000000 - 01111111 = 0 - 127
B = 10000000 - 10111111 = 128 - 191
C = 11000000 - 11011111 = 192 - 223
D = 11100000 - 11101111 = 224 - 239
E = 11110000 - 11111111 = 240 - 255

The numbers that are 15 apart are just coincidence in working with the binary scale.