# Subnetting question

• posted

Okay, I got my CCNA a couple months ago and am going toward my MCSE now....however, I haven't dealt with subnetting since my course material in Cisco so please forgive me .....

I have the McGraw/Hill MCSA/E 70-291 book and am covering subnetting. I haven't dealt with subnetting in a while....here's my question (straight from the book)

If you have an IP of 10.10.1.0 /23 how many subnets does this give you?

Well, wouldn't you take 2 to the power of 7 giving you 128? The book says the correct answer is 32,766.

How in the world do you get 32,766 when the 3rd octet of the mask has

7 networking bits?

I'm either way off base or the book has had 3 wrong answers when it comes to subnetting which I find hard to believe. Please help me out here.

• posted

Since 10.10.1.0 /23 is a Class A network, then I really only have 15 bits to play with (since the first octet is sorta locked b/c the default netmask for Class A is 255.0.0.0)...therefore you'd take 2^15 and get 32768 (if you can't use subnet zero you'd get 32766)

Am I right? I know I am...but I need someone to lock it in stone for me please :)

• posted

That is correct. Class A network means the first octek is the network and next 3 are the host. So using a /23 gives you 15 bits for the subnet address and 7 for the host. Since you're not using subnet zero you get...

Networks .. (2^15)-2 = 32766 Host ...... (2^7)-2 = 126

Terry

• posted

Host ........ (2^9)-2 = 510

;-)

• posted