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To truly understand how to derive IP masks and apply them to addresses, you must understand binary numbers and how to convert them to decimal. Let's start with something that we're all pretty comfortable with, namely decimal base 10 numbers. Back when we were kids, we were taught that each digit in a decimal number stood for a different power of The number , for example, is interpreted as follows:.
Now this is pretty simplistic, I admit, but understanding this is the basis for understanding any numeric base. In particular, it will help us understand binary base 2. We interpret binary numbers in exactly the same way as decimal numbers, except that each column of a binary number represents a different power of 2 rather than We can easily convert a binary number to a more understandable decimal value.
Let's first review the powers of 2 we're only going to go as far as we need to for an 8-bit byte because IP addresses have 8-bit bytes. Now, we can apply what we know about binary numbers to IP addresses and subnet masks. IP addresses are 32 bits, or four 8-bit bytes, in length. While the computer stores the IP address in binary, we typically use dotted decimal notation to write out addresses because we find it easier to read.
Dotted decimal notation lets us examine an IP address one byte at a time. Subnet masks, like the IP address itself, are 32 bits in length. With classful addressing, then, the subnet mask will have 8, 16, or 24 one bits for Class A, B, and C addresses, respectively.
In the parlance of subnet masking, these masks would be said to be 8, 16, or 24 bits in length but that is a misnomer; it really only refers to the number if one bits since masks really are always 32 bits long.
Variable length subnet masking VLSM is essential to support classless addressing. VLSM allows us to build masks that are of pretty much any length and are not restricted to the byte boundaries of classful addressing. Let's start with a simple example. Suppose we have the Class C address In binary, the address with spaces inserted for readability is:.
But how does this really work? So let's carry out that operation for the Class C address and mask above:. Let's now try a broader example. Since masks are created by writing some number of ones followed by zeroes, an all-ones byte will have the value and an all-zeroes byte will have a value of 0, as shown above. But a VLSM may not have a mask that falls on a byte boundary so one of the bytes may have a value other than 0 or In fact, an 8-bit byte has only eight possible subnet values as we increase the number of one bits from the left:.
Variable-bit subnet masks give us a great deal of flexibility in carving out multiple subnets within the Class C space. Suppose that we want to create eight subnetworks in the We just add 3 bits to the length of the bit subnet mask. Recall that the first 24 bits are all ones, so the first three bytes will be The fourth byte will have 3 ones in it and, therefore, a value of from the table above.
Because we used 3 bits of the final byte as a mask sometimes called a subnet ID , the host IDs are limited to 5 bits. But we still have one more significant problem to solve, namely, to identify the subnet numbers. The eight possible values of the 3-bit subnet mask are:.
Therefore, the eight possible values of the final address byte are again, the spaces are only for readability:. For obvious reasons, you should always indicate the subnet mask along with the address itself, as I've done above, to avoid ambiguity; the address You can reach him at gck garykessler.