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| justindu 2003-10-02, 8:53 am |
| Given the subnet mask 255.255.128.0, how many subnets can address randge 11.0.0.0 support?
The answer is 512.. but why dont we count the bits in the first octet when configuring the number of available subnets?? | |
| edmonds_robert 2003-10-02, 9:26 am |
| Because when you're talking about subnetting, you are talking about the classful network, which, in this case is 8 bits long. | |
| OHCCNP2003 2003-10-02, 11:11 am |
| Think about the default subnet mask for Class A - 255.0.0.0
This mask uses 8 bits for the subnet portion and 24 bits for the host portion of the address. The default mask gives you 1 subnet with 16777214 hosts.
To allow more subnets to be created, you borrow from the host portion.
Your mask borrows 9 bits from the host portion of the mask leaving 15 available.
The value of your borrowed bits is 2 to the ninth power which equals 512. For Cisco purposes, subtract 2. This leaves you with 510 (not 512) subnets and 32766 hosts per subnet. | |
| bloodshotx 2003-10-02, 2:24 pm |
| make sure you understand this concept before moving on.
CIDR and VLSM will just confuse you even more if you don't. | |
| justindu 2003-10-02, 5:16 pm |
| ok.. i am grasping it more and more.. my test is 2 days from now so i better! 
I just get confused with the class a type addresses that use a class c mask! | |
| OHCCNP2003 2003-10-02, 9:04 pm |
| Am attaching an example of a problem involving a Class A address. Hope this helps. | |
| justindu 2003-10-02, 10:03 pm |
| thank you so much |
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