Given the limited time you have to dispatch questions in the structured environment of a Microsoft certification exam, every shortcut to coming up with the correct answer is a plus. The following method, using CIDR notation, can shave minutes off the time it takes you to complete a single question. Since you already understand the underlying binary technology at the heart of subnetting, you can use the following shortcuts, one for each address class, to come up with the correct answer without working in binary.
Identifying Class C Subnet Characteristics
Consider the host address 192.168.10.50/27. The following steps flesh out the details of the subnet of which this address is a member:
- Obtain the CIDR-n otation prefix length for the address by converting the dotted- decimal mask to CIDR notation.
In this case, /27 corresponds to a mask of 255.255.255.224. Practice converting between these notations until it becomes second nature.
2. Using the closest multiple of 8 that is greater than or equal to the prefix length, compute the interesting octet (the octet that increases from one subnet to the next in increments other than 1 or 0). Divide this multiple by 8. The result is a number corresponding to the octet that is interesting.
In this case, the next multiple of 8 greater than 27 is 32. Dividing 32 by 8 produces the number 4, pointing to the fourth octet as the interesting one.
3. To compute the incremental value in the interesting octet, subtract the prefix length from the next higher multiple of 8, which in this case is 32. The result (32 – 27) is 5. Raise 2 to the computed value (25 = 32). The result is the incremental value of the interesting octet.
4. Recall the value of the interesting octet from the original address (50 in this case).
Starting with 0, increment by the incremental value until the value is exceeded. The values then are 0, 32, 64, and so on.
5. The subnet in question extends from the increment that is immediately less than or equal to the address’s interesting octet value to the address immediately before the next increment. In this example, 192.168.10.50/27 belongs to the subnet 192.168.10.32, and this subnet extends to the address immediately preceding 192.168.10.64, which is its broadcast address, 192.168.10.63.
Note that if the interesting octet is not the fourth octet, all octets after the interesting octet must be set to 0 for the subnet address.
6. The usable range of addresses for the subnet in question extends from one higher than the subnet address to one less than the broadcast address, making the range for the subnet in question 192.168.10.33 through 192.168.10.62. As you can see, 192.168.10.50/27 definitely falls within the subnet 192.168.10.32/27.
Identifying Class B Subnet Characteristics
Using the steps in the previous section, find the subnet in which the address 172.16.76.12 with a mask of 255.255.240.0 belongs.
- The corresponding CIDR notation prefix length is /20.
- The next multiple of 8 that is greater than 20 is 24. 24/8 = 3. Octet 3 is interesting.
- 24 – 20 = 4, so the incremental value is 24 = 16.
- The increments in the third octet are 0, 16, 32, 48, 64, 80, and so on.
- The increments of 64 and 80 bracket the address’s third-o ctet value of 76, making the subnet in question 172.16.64.0, after setting all octets after the interesting octet to 0. This subnet’s broadcast address is 172.16.79.255, which comes right before the next subnet address of 172.16.80.0.
- The usable address range then extends from 172.16.64.1 through 172.16.79.254.
Identifying Class A Subnet Characteristics
Try it one more time with 10.6.127.255/14. Combine some of the related steps if possible:
- The prefix length is 14. The next multiple of 8 that is greater than or equal to 14 is 16. 16/8 = 2, so the second octet is interesting.
- 16 – 14 = 2, so the incremental value in the second octet is 22 = 4.
- The corresponding second- octet value of 6 in the address falls between the 4 and 8 increments. This means that the subnet in question is 10.4.0.0 (setting octets after the second one to 0) and its broadcast address is 10.7.255.255.
- The usable address range is from 10.4.0.1 through 10.7.255.254.
Determining Quantities of Subnets and Hosts
The general technique described in the previous sections is also useful when trying to determine the total number of subnets and hosts produced by a given mask with respect to the default mask of the class of address in question.
For example, consider the Class B address 172.16.0.0 with a subnet mask of
255.255.254.0. This is a prefix length of 23 bits. When you subtract the default prefix length for a Class B address of 16 from 23, you get the value 7. Raising 2 to the 7th power results in the value 128, which is the number of subnets you get when you subnet a Class B address with the 255.255.254.0 mask.
Determining the number of hosts available in each of these 128 subnets is simple because you always subtract the prefix length that the subnet mask produces, 23 in this example, from the value 32, which represents the total number of bits in any IP address. The difference, 9, represents the remaining number of 0s, or host bits, in the subnet mask. Raising 2 to this value produces the total possible number of host IDs per subnet that this subnet mask allows. Remember to subtract 2 from this result to account for the subnet and broadcast addresses for each subnet. This gives you the actual number of usable host IDs per subnet. In this case, this value is 29 – 2 = 510.
Repeated practice with this technique will reduce your time to obtain the desired answer to mere seconds, leaving time for the more challenging tasks in each question. You have a wealth of examples and scenarios in this chapter, as well as in the review questions, on which to try your technique and build your trust in this faster method.
Supernetting
Let’s take a look at a different type of subnetting. Class B addresses give you 65,534 addresses, but let’s say that you have 1,000 users. Would you really need a Class B address?
Not if you use supernetting.
Supernetting allows you to have two or more blocks of contiguous subnetwork addresses. So, what does that actually mean? Class C addresses give you 254 usable addresses. So, if you needed 1,000 users, you could set up supernetting of four Class C addresses that are contiguous:
Example:
192.168.16.0
192.168.17.0
192.168.18.0
192.168.19.0
When you set up supernetting for a Class C, you would use a Class B subnet mask. When you set up supernetting for a Class B, you would use a Class A subnet mask. This allows you to use multiple classes to get a larger number of hosts without taking up an entire class.
So, the subnet mask for the previous example would be 255.255.252.0, or /22. The reason we used this subnet mask is because a 252 subnet mask allows for four subnets. Each of those Class C numbers would equal one subnet on this network.