IP Routing Primer: Part One | Page 2

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Storage & Servers

IP Routing Primer: Part One

November 6, 2000
by Peter Morrissey

Table of Contents

Subnet Masks

The subnet mask is a required setting on any host that communicates on an IP network. It is commonly represented in the same format as an IP address, except that the values usually are either 255 or 0. (Exceptions to this are becoming more common--we'll talk about those later). If you've ever seen a subnet mask, you will notice that a series of one or more 255 values are always designated in succession, followed by a series of 0 values. If you line a subnet mask up over an IP address by the dots, each number below a 255 will be part of the network address. The remaining numbers under the 0s will be the node address.

255.255.255.0	= mask
128.230.100.4	= IP address

With this type of subnet mask, the boundary between the network and node address is always on a dot between the last 255 and the first 0, sometimes referred to as the byte boundary.

What really matters to a computer of course is the binary representation. The above mask and IP address are represented in the following manner as binary:

11111111.11111111.11111111.00000000 Mask

10000000.11100110.01100100.00000100 IP Address

With the binary representation, an uninterrupted sequence of 1s in the mask, line up exactly over the subnet portion of the address. With this type of mask, seeing the binary version doesn't add any value, but later it will be necessary to see the binary data to fully understand masks that don't end neatly on one-byte boundaries.

Part of the decision as to how to set up the subnet mask is left to the network designer, but the subnet mask must be configured consistently throughout the network. Another factor that limits the definition of a subnet mask is its class. For example, with a "Class B" address, the first two bytes always have to be included in the network portion of the address, and thus the subnet mask will always start out with 255.255 or 11111111.11111111. With a "Class C" address, the first three bytes have to always be included in the network portion of the address. In this case the subnet mask would have to start with 255.255.255.

In each instance, there is a lot of flexibility in the way that the rest of the address is masked. The example above is a Class B address. The network address is 128.230.100. The node address is 4. The example above used a subnet mask of 255.255.255.0. This makes it possible to have 254 unique, routed networks or subnets because the mask can include the third byte in the network address. (You can't use 255 because that's reserved for broadcast address, and 0s, used in an older style of broadcast address, can't be used either; 0s at the end of an address are also used to symbolize network addresses.) Even though the first two bytes cannot be changed, the third byte can have any value from 1 to 254. This means that with a three-byte or 24-bit subnet mask, the following distinct networks or subnets are possible with the address:

128.230.1.0

128.230.254.0

There is one byte left over for node addresses, so each network or subnet will have 254 node addresses. If the minimum subnet mask of 255.255.0.0 were used, there could only be one routed network. But two bytes would be left over for node addresses. So the range of possible node addresses would be 128.230.1.1 to128.230.254.254 or approximately 65,000 nodes. The disadvantage of this is that you cannot divide your network into routed subnets.

With this particular address, if you divide only the subnet mask on the byte boundaries, you have 254 subnets limited to 254 nodes each, or you have one big network with 65,000 nodes. These two extremes have some obvious trade-offs. There may be cases where you would want more than 254 nodes on a routed subnet, but the other extreme flattens the network completely, giving you no ability to divide your network into routed subnets.

Fortunately, the subnet mask doesn't have to divide the network address and the node address on a dot boundary. In some cases it can be divided somewhere in between. When the network address and node address are not divided on a dot boundary, the subnet mask is called a variable-length subnet mask. A variable-length subnet mask is commonly represented in decimal form, such as 255.255.254.0. But its binary representation has to be considered to understand where the network address ends and the node addresses can begin. The following example uses a subnet mask of 255.255.254.0:

255.255.254.0 = mask

128.230.00.2 = IP address

To understand this, we'll have to look at it in binary form. Here the mask does not line up on the byte boundaries:

255.255.254.0 = 11111111.11111111.11111110.00000000 Mask

128.230.004.2 = 10000000.11100110.00000100.00000010 IP Address

One thing you should notice is that there are contiguous 1s from left to right in the subnet mask. This is one of the rules of a subnet mask: All the binary 1s have to be contiguous, and all the following 0s overlaying the node address have to be contiguous. A mask that doesn't follow this pattern is not valid. In any case, the valid node addresses are all possible numbers that can be represented under the 0s, so the lowest number is going to be: 128.230.4.1 or 10000000.11100110.00000100.00000001. And the highest number is going to be 128.230.5.254 or 10000000.11100110.00000101.11111110. You will also notice that this mask has one less one bit than the previous mask. This is sometimes referred to as a 23-bit mask.

11111111.11111111.11111110.00000000 = 255.255.254.0 = mask

10000000.11100110.00000100.00000000 = 128.230.004.0 = network address

10000000.11100110.00000100.00000010 = 128.230.004.2 = node address

10000000.11100110.00000100.00000001 = 128.230.004.1 = lowest possible node address

10000000.11100110.00000101.11111110 = 128.230.005.254 = highest possible node address

Because subnet masks have to be contiguous 1s followed by contiguous 0s, the last byte of a subnet mask can only have the following binary or decimal values: