Going For CCNA, have a "simple" question 4

[ts8586]

Simple for some, but not for me!!!! Anyway, I'm going through Todd Lammle's CCNA Study Guide, and in the subnetting chapter, he gives examples about subnetting Class C subnet 0. I'm guessing he means 255.255.255.0??? Is that correct?

He also mentions Subnet 64. Does he mean 255.255.255.192? Any clarifications would be appreciated. Thanks.

 

[burtsbees]

Subnet zero means, for example, 192.168.1.0/24 let us say is broken down into subnets...not necessarily VLSM...let's say /28...
192.168.1.0
192.168.1.16
192.168.1.32
192.168.1.48
and so on as they repeat every 16
Subnet zero in this case is 192.168.1.0

Burt

 

[ledzepp817]

I was confused on this at first as well.

Most CCNA books say that when you subnet a network, you can't use the subnet zero or the highest subnet either. In the CCNA books, it will say that the total number of subnets is 2^n - 2 where n is the number of subnet bits you borrow from the host bits. In reality, this isn't true. But for the CCNA test, it is!

Example:
lets say a class C network 192.168.1.0, with a 255.255.255.192 mask. There are two subnet bits, so total number of subnets according to CCNA exam is 2 (4 - 2...DUH!), the subnets are 192.168.1.64 and 192.168.1.128. CCNA says you can't use the 192.168.1.0 or the 192.168.1.192 subnets. But in reality, you can use these if you use the "ip subnet-zero" global configuration command, which I think is enabled by default now.

Hope this helps!

 

[CiscoGuy33]

ts8586,

Just be aware that as ledzepp817 said about the "ip subnet-zero" command - I have seen practice questions where they ask about the number of subnets and then say whether or not that command has been used.

Every exam question I ever saw from Cisco - they always made it very clear if "ip subnet-zero" was being used or not!

Hope this helps!


E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[lerdalt]

This is what helped me understand the subnet zero as well as subnetting in general, hopefully it will help out someone else.

When counting (whether it's base2 (binary), base8 (hex), or base10, you always start counting at 0. Look at a quick easy /24 mask....range is x.x.x.0 - 255. With this sort of thinking, the subnet zero is the first available subnet that can be used.

 

[ts8586]

Thanks for the tips, everyone. This stuff is starting to click... I think (reading Todd Lammle's chapter over and over). Just to make sure, if I have a subnet of 192.168.10.32 and a broadcast address of 192.168.10.63, is my valid range .33 to .62?

 

[ledzepp817]

Yes, your mask would be 255.255.255.224 or a /27. Your usable host IP range is 192.168.10.33 - .62

The network address is 192.168.10.32.
Broadcast is 192.168.10.63.

 

[Gharus]

On the test, they will specifically say that IP-SUBNET-ZERO is enabled if they want you to use it. Otherwise, assume that IP-SUBNET-ZERO is not enabled, even though it is enabled by default after IOS version 11.3 (I think that is the right version).

 

[ts8586]

Rather than start a new thread, just thought I'd let you all know that I passed today with a 927. Thanks again to everyone who answered my "dumb" questions. It definitely helped me in studying the areas that I was weak in. Glad to get this one done, now on to VPN's!

 

[burtsbees]

Congrats!

Burt

 

[CiscoGuy33]

ts8586,

GREAT JOB


E.A. Broda
CCNA, CCDA, CCAI, Network +