|  | 10.7 | Creating a Subnet
| 10.7.3 | Computing subnet mask and IP address
Whenever you borrow bits from the host field, it is important to note the number of additional subnets that are being created each time you borrow one more bit. You have already learned that you cannot borrow only 1 bit; the fewest you may borrow is 2 bits. Borrowing 2 bits creates four possible subnets (22) (but you must always remember that there are two reserved/unusable subnets). Each time you borrow another bit from the host field, the number of subnets created increases by a power of 2. The eight possible subnets that are created by borrowing 3 bits is equal to 23 (2 x 2 x 2). The sixteen possible subnets created by borrowing 4 bits is equal to 24 (2 x 2 x 2 x 2). From these examples, it is easy to see that each time you borrow another bit from the host field, the number of possible subnets doubles. Question: How many bits are being borrowed (how long is the subnet field) for a Class B network using a subnet mask of 255.255.240.0? Answer: The first two octets of the mask (255.255) correspond with the 16 bits in a Class B network number. Remember that the subnet field is represented by all the additional "1" bits past that. The number 240 decimal is 11110000 in binary, and you can see that you are using 4 bits for the subnet field. Question: How many possible subnets are there with a 4 bit subnet field? Answer: Start with finding the smallest 4 bit number - 0000 - then the largest 4 bit number - 1111 (15). So the possible subnets are 0-15, or sixteen subnets. However, you know you cannot use subnet 0 (it's part of the network address), and you cannot use subnet 15 (1111) either (broadcast address). So this 4 bit subnet field gives you fourteen usable subnets (1-14).
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What's a Netmask?