Calculating # of subnets per network???

Noods

I have seen two methods of finding the number of subnets per network.

2(# of subnet bits - 2) and 2(# of subnet bits)

The above equations are "to the power of" not simply multiplied. All guides list the first method as deprecated. Can I assume the CCNA tests will us the second method for finding usable subnets?

EdTheLad

Noods wrote:

I have seen two methods of finding the number of subnets per network.

2(# of subnet bits - 2) and 2(# of subnet bits)

The above equations are "to the power of" not simply multiplied. All guides list the first method as deprecated. Can I assume the CCNA tests will us the second method for finding usable subnets?

Hi Noods,
Originally Cisco did not recognise subnet zero and the last subnet.
With newer releases of the IOS the command Subnet Zero is now by default on all routers.Therefore its the second answer.
This was a very stupid mistake and in my opinion im very glad it has been fixed.
Older versions of the CCNA used the -2 so if your studying old doc's you will see this confusion.

nothing_pt

But in the exam they say what method to use or not?

EdTheLad

nothing_pt wrote:

But in the exam they say what method to use or not?

Yes, in the exam do as follows.

10.0.0.0 mask 255.192.0.0
subnet bits are 2 therefore subnets are 2 to the power which equals 4.

Valid subnets are 0,64,128,192

In the older exams this would have been incorrect as they didnt account for 0 and 192.
In 640-801 now that they mention subnet zero they require all valid subnets which are mentioned above.

porengo

ed_the_lad wrote:

nothing_pt wrote:

But in the exam they say what method to use or not?

Yes, in the exam do as follows.

10.0.0.0 mask 255.192.0.0
subnet bits are 2 therefore subnets are 2 to the power which equals 4.

Valid subnets are 0,64,128,192

In the older exams this would have been incorrect as they didnt account for 0 and 192.
In 640-801 now that they mention subnet zero they require all valid subnets which are mentioned above.

Questions pertaining to subnets on the current CCNA exam will specify if subnet zero is being used or not. Don't automatically assume that subnet zero is being used. Read the question carefully before answering.

nothing_pt

Thanks ppl.

I intend to do it next week, but the more i study the insecure i feel. Bah!

Noods

Thanks all!

normt

...by including subnet zero you mean to include both the subnet with the network address with all zero bits & the subnet with the broadcast address of all 1 bits?


Out of interest, why would Cisco chose to include these, how can they be used?

porengo

Smoggy wrote:

...by including subnet zero you mean to include both the subnet with the network address with all zero bits & the subnet with the broadcast address of all 1 bits?


Out of interest, why would Cisco chose to include these, how can they be used?

IPv4 was scalable for a time before it resulted in an inefficient allocation of addresses. In other words, we are running out of public addresses. IPv6 was developed to resolve that problem with virtually unlimited address space, and some networks have begun to implement it. But, over the past two decades, engineers have successfully modified IPv4 so that it can survive the growth of the internet.

VLSM (Variable Length Subnet Mask) was developed to gap the bridge between IPv4 and IPv6. VLSM allows a network administrator to use more than one subnet mask within the same network address space. And maximizes address effificency that is often referred to as "subnetting a subnet," or "super subnetting." VLSM uses protocols such as RIPv2, EIGRP, OSPF and IS-IS. Static routing can also be used.

In the past, the use of subnet zero was discouraged because of the confusion that could occur if a network and a subnet had the same address. This is also true for the "all-ones" subnet. However, Cisco IOS version 12.0 or later allows you to use subnet zero by default. It gives a network administrator the option of designing an addressing scheme that allows for growth and doesn't waste addresses. But keep in mind, not all routers allow the use of subnet zero.

I hope this clears it up for you!

normt

Got you now 100%, I never even considered vlsm.

much appreciation for your prompt explanation.

porengo

No problem. Glad I could help.