| reply to Questionforyou
Ow! Ow! Ow! Ow!
Just memorize your subnets! It is really easy, and will give you a HUGE advantage on your CCNA test when you can answer a subnetting question in 2 seconds thus affording you more time for the Sims.
Sit down for 15 minutes a day and chant:
/25 is a .128 mask with a block size of 128, 126 hosts and a wild-card mask of .127. One bit on, 7 bits off
/26 is a .192 mask with a block size of 64, 62 hosts and a wild card mask of .63. Two bits on, 6 bits off
/27 is a .224 mask with a block size of 32, 30 hosts and a wild-card mask of .31. Three bits on, 5 bits off
/28 is a .240 mask with a block size of 16, 14 hosts and a wild-card mask of .15. Four bits on, four bits off
/29 is a .248 mask with a block size of 8, 6 hosts and a wild-card mask of .7. Five bits on, three bits off
/30 is a .252 mask with a block size of 4, 2 hosts and a wild-card mask of .3 Six bits on, 2 bits off.
Then when you need to subnet a Class B, subtract 8 from all of these, or 16 for a class A. Think about it for a while and it makes sense.
For Class A & B number of hosts & block size, memorize your powers of 2 up-to 32768. Really, just know them. Sit down for another 15 minutes a day and chant through them.
Then the simple formula of 2^x = number of subnets, 2^y-2 = number of hosts per subnet where x= masked bits, y = host bits.
For example, a Class B address 172.17.20.0 with a .224 mask:
You know that .224 is 3 masked bits from your memorization. so 2^3 = 8 subnets with (2^13)-2 (remember to add all of the trailing host bits 5+8) 8192 = 8190. Your block size is 32 (memorized) so now just apply the information to the relevant octet (which class you are in).
172.17.0.0 - first network address| 172.17.32.0 - second network address
172.17.0.1 - First valid host
172.17.31.254 - last valid host (broadcast - 1)
172.17.31.255 Broadcast address (Next octet -1)
And there you have subnetted a class B in your head. |
