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IP Routing Primer: Part One

November 6, 2000
by Peter Morrissey

Table of Contents

Anatomy of an IP Address
Subnet Masks
Broadcast Addresses

In many ways, the task of a router is very simple. It must look at the destination address of an IP packet and then forward the packet to another router or to a directly connected host. However, if youıve ever attempted to get two devices to communicate on an IP network, you know that it doesnıt always work.

In this Network Design Manual article weıll show you the steps involved in getting a packet from Point A to Point B on an IP network. Weıll talk about the settings that are necessary on a host to communicate on a routed network and what they really mean. Once you understand this, youıll be able to ask the right questions when things donıt go as expected and even do a little bit of your own troubleshooting.

Anatomy of an IP Address

When one host has data to send to another host, it sends it to its local IP process, which builds an IP packet for transmission on the network. The packet has a header, which includes the destination IP address as well as the source address of the local host. The destination and source IP addresses stay with the data until it arrives at the host so that all the routers it traverses know in what direction to send it. The source address provides the destination host with an address to which it can send its response.

To understand how this happens requires a deeper knowledge of the IP address. It's obvious that an IP address consists of four separate numbers, separated by dots. Each number represents one byte worth of data, limiting it to a range of 256 possible values, commonly represented as decimal values from 0 to 255 or binary values from 00000000-11111111.

One thing that is not so obvious is that each IP address is always divided in to two sections. The first section always delineates the network address, or subnet. This is the part routers are primarily concerned with. The second section, combined with the first section forms a unique node address. The subnet mask is configured to tell the host or client which part is the network address and which part is the node address.

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:

11111111	= 255
11111110	= 254
11111100	= 252
11111000	= 248
11110000	= 240
11100000	= 224
11000000	= 192
10000000	= 128
00000000	= 0

Broadcast Addresses

There are times when a device has to send an IP packet to every node on a network. In such an instance, it will use the broadcast address of that network. The subnet mask is also used to derive the broadcast address. This is done by replacing all of the binary digits in the node address, as governed by the trailing 0s in the mask, to values of binary ones. This is combined with the network address, which results in the broadcast address. Following is the broadcast address that would go with our above example.

11111111.11111111.11111110.00000000	= 255.255.254.0	= mask
10000000.11100110.00000100.00000000	= 128.230.004.0	= network address
10000000.11100110.00000101.11111111	= 128.230.005.255	= broadcast address

Broadcast addresses also are easier to figure out when the masks end on the one-byte boundaries. When this is the case, it can easily be represented in decimal by substituting decimal 255s in place of the bytes where the mask indicates the node address belongs. Here's an example: