I am more of a math guy, so let me tell you my trick to figuring out subnetting. It requires no calculators, no pieces of paper, and no converting to binary. N is the number of hosts or networks you are trying to subnet, while X is the number of bits in the subnet mask you need to reserve.
X must be a whole number, and it is rounded up. Notice the minus two part of the formula. This accounts for the broadcast address and subnet mask. You have been asked to create a subnet that allows for 3 usable addresses that has the largest number of subnets possible. Now ask yourself, 2 to what power equals 5? Remember, that X must be a whole number rounded up. The concept of using subnet zero which is on by default with Cisco now is different than being allowed to use the subnet identifier address.
Possible subnets are: As we can see, in the All-Ones-Subnet they are all set to the value one. Traditionally it was strongly recommended avoiding the use of all-zeros-subnet and all-ones-subnet for addressing , as indicated in the RFC where the subnetting rules are defined.
Besides recommendations, the whole IP addressing space has always been usable so it was the System Administrator responsibility ensuring the correct use. The RFC states that all modern software need to be able to use all the possible subnets, so there no more necessity of avoiding the all-zeros-subnet and all-ones-subnets.
Modern routing protocols do not have a problem with the all-zeros and all-ones subnets. The use of the subnet zero for addressing was discouraged because of the confusion when having a network and a subnet with indistinguishable addresses. The use of the all-ones subnet for addressing was discouraged in the past because of the confusion when having a network and a subnet with identical broadcast addresses.
Modern hardware has no problem with the all-zeroes or all-ones subnets, but some very old hardware might get you in trouble. As we can see above, no errors when using the subnet zero on a loopback interface. You can find detailed explanation about Subnet Masks in this article. The all-zeros and all-ones subnets are the typical tricky questions in network certification exams.
When preparing the test, you need to make sure you investigate how the all-zeros and all-ones subnets are treated in that particular certification path. For example, in a subnetting calculation question the restrictions about all-zeros and all-ones do not normally apply, since the subnetting is basically a math exercise. But if the question is asking about the use of those specific addresses on a router, then you need to think twice before answering.
Normally the best way is checking the blueprint of the certification for which you are studying. You can check if all-zeros and all-ones subnets are covered, and investigate if their use is allowed in router configuration questions. We have presented the all-zeros and all-ones subnet problem, and the implications that might arise in using them in a real network.
According to RFC , "It is useful to preserve and extend the interpretation of these special network and broadcast addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual physical subnets. The -2 takes into account that subnet zero and the all-ones subnet are not used traditionally. Using subnet zero for addressing was discouraged because of the confusion inherent in having a network and a subnet with indistinguishable addresses.
With reference to our example above, consider the IP address If you calculate the subnet address corresponding to this IP address, the answer you arrive at is subnet Note that this subnet address is identical to network address This was formerly a source of great confusion.
However, if a network engineer working with a Cisco IOS software release older than Use of the all-ones subnet for addressing has been discouraged in the past because of the confusion inherent in having a network and a subnet with identical broadcast addresses. With reference to the example above, the broadcast address for the last subnet subnet In other words, a network engineer could configure the address Although the all-ones subnet can now be used, misconfigurations can cause problems.
To give you an idea of what can happen, consider the following:. Note: See Host and Subnet Quantities for details. Routers 2 through 5 are access routers that each have several incoming asynchronous or ISDN connections.
We have decided to break up a network Each piece is given to one of the access routers. Also, the asynchronous lines are configured ip unnum e0.
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