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Page last modified:
05/05/11 10:30 AM
Basic Subnetting Concepts
There are typically three types of questions on certification tests.
Configuration/troubleshooting questions.
Combination Questions--just complete the planning steps then the configuration/troubleshooting steps
Planning questions (back to top)
You typically have to calculate a subnet mask based on either the number of hosts required or the number of networks required. The subnet mask consists of 32 bits in binary that are either 1's or 0's. The combination of 1's and 0's MUST equal 32.
To answer a planning question based on the number of hosts you need simply use the following formula:
For example: To determine a subnet mask that requires at least 2000 IP (or host) addresses:
Solve for N
For this example, you will need 11 zero's in your subnet mask . Since 32-11=21, then 21 ones are needed . Your new subnet mask will be:
11111111.11111111.11111000.00000000 Convert to decimal 255.255.248.0
To answer a planning question based on the number of subnetworks you need use this formula
For example: Supposed you were assigned a class C IP address and needed to break it down into 4 networks. The default subnet mask for class C is /24 or 11111111.11111111.11111111.00000000
Solving for N we get 2. That means we borrow two bits from the host portion of the mask.
This means our new subnet mask is /26 or 11111111.11111111.11111111.11000000
Convert to decimal: 255.255.255.192
Recap: To answer a planning question, simple do the math. It works EVERY time!
Configuration/troubleshooting questions (back to top)
These type of questions typically deal with things like:
What is the broadcast IP for a segment, what is the first valid IP address in a segment, or what would the IP address of a host in the same segment as the host with this IP address (w.x.y.z)
To answer these types of questions, you simply determine what Cisco calls the block size. Your networks are always multiples of this number.
For example: for a /27 subnet mask or 11111111.11111111.11111111.11100000, determine the decimal value of the right most one in the subnet mask. Note which octet is subnetted. We are only concerned about that octet. The fourth octet is displayed below. It is VERY important to know the octet you are subnetting in to answer exam questions.
The block size is 32. This means the networks are always multiples of 32.
Shortcut: If the subnet mask is expressed in decimal, just subtract the mask in the subnetted octet from 256 to determine the block.
Since we are subnetting in the last octet the networks are:
Calculate the range of IP addresses, the network ID or the broadcast IP to answer any question.
Example
A 28 bit mask means we are subnetting in the fourth octet.
The value of the right most 1 in the mask is 16 so our networks are always multiples of 16
We can see the 210.10.5.68 address falls in the 64 network. This makes the correct answer C
Important note: Some Cisco questions do not allow the use of IP Subnet 0. If that is mentioned in the question, you cannot use the first available subnet. It the above example, our networks would strt with 210.10.5.16