By Garrett Seeley and Clinton Williams
In previous columns, we discussed the importance and usage of the OSI model and the settings of TCP/IP, specifically the IP address itself. However, we did not explore a complex side to IPv4 – the subnet, also called the subnetting, the subnet address or the subnet mask. We discussed how a subnet draws a line in the IP and helps to differentiate between the network and the host portion of the IP address. However, we glossed over an important part of IPv4 which is that the subnet address can contain a number other than 000 or 255. This is when technicians use subnetting.
Subnetting, in its simplest form, is the art of slicing an IP space into smaller pieces. Consider it dividing a pie into four slices and dedicating each section to a specific program or department. In a hospital setting, the IT department manages the pie and slices IP space into smaller chunks to isolate networks, manage network traffic, bandwidth, and monitor other metrics. For example, it’s good practice to keep radiology traffic isolated from cardiology traffic. That way the network will run more efficiently and have less congestion. Instead of having a single IP space support both departments, IT would use subnetting to slice the space into two subnets, isolating each department into its own space. The network traffic has less interference, and this helps to implement traffic controls. This is a preferred method of networking in a hospital because of the increasing number of machines on a network.
Several residential networks use a simple subnet. This is what most technicians are used to because it is what they have at home. In this type of network, there is one subnet and all the machines talk on the network in the same subnet. Because of the large number of hosts in a hospital, this is not realistic. It would be like talking at a noisy sporting event. There would simply be too much traffic for the hosts to communicate effectively. To improve on this, TCP/IP separates larger networks into smaller sub netted networks using the subnet mask. In this way, hosts do not compete with traffic outside of their departmental function. To help understand this, recall that a subnet of 255.255.255.000 contains about 250 hosts, whereas a subnet of 255.255.000.000 gives over 65,000 hosts. This limitation on the number of hosts is set by the subnet in a binary number format. Recall that 255.255.255.000 is 11111111.11111111.11111111.00000000 in binary. We call this a /24 subnet because there is a total of twenty-four “1’s” in the subnet. The slash is just a shorthand for the binary address of the subnet mask. In the previous articles, we spoke of drawing a line after the change in the subnet. That rule works if the subnet is written in binary or in decimal. Unfortunately, this does not give us enough hosts for larger hospital departments, say for example one using over 300 infusion pumps. Let’s explore what happens when technicians need a bigger network, or the subnet has something other than 255 or 000 in the octets.
Consider an unusual subnet of 11111111.11111111.11111110.00000000. This is a /23 network. Notice there is still only one line in the subnet. In decimal form, it is 255.255.254.0. In this different from the previous subnet as the line has moved to the second least significant binary digit in the third octet, second from the right. There is still only one line, however, the IP will not cross this line unless it uses a part of the third octet. For those that like math, crossing from host to network is clearer if the IP addresses are drawn out like the subnet mask in binary math. For further clarification, remember that this is also how we see hexadecimal addresses such as a MAC address or IPv6. In binary, it will be clearer when the IP binary number crosses least significant digit of the subnet mask. For this article, we will not go through that exhaustive process. There is a clear way to see the number of hosts in a subnet.
Imagine for a moment if each of the octets were a number and the subnet told you how big the network is by multiplying its number to the previous octet. For this math, use the table below. In this table, a subnet of 255.255.254.000 would give 1 x 1 x 2 x 255 hosts, for about 512 hosts. If more hosts are needed, change the subnet. A subnet of 255.255.224.000 gives 1 x 1 x 32 x 255, or 8160 hosts. This is plenty of space for a larger scale network. In addition, the hosts will know how to talk to the machines outside their subnet by using the LAN router. Network traffic will naturally segment along the subnet mask by using the dividing line between the host and network IP address. In this way, the subnet separates traffic and reduces congestion.
Sometimes it’s important to find out how many subnet networks are in the subnet mask. This is represented by counting the unused part of the subnet mask in binary. Count the 1’s from the right to the left. If that were a binary number, what would the number be? Again, this is fun for people who love doing binary math. For the rest of us, here is a chart to figure it out. Finally, for those that are hopelessly lost, there is one saving grace. Go online and look up the phrase “Online Subnet Calculator” That is ultimately what most network professionals do when they say “Well … it’s a /20 network.” Yes, there is literally an app for that. Get the app and compare the numbers here to it. Look for patterns and subnetting should become clearer.
Ultimately, subnetting is an important element of networking in that it allows technicians to use larger groups of hosts, connecting devices with a similar function in a large network. It is scalable, manageable and segmented. Subnets can be isolated, and the traffic monitored from each other. For these reasons, most hospital settings use a sub netted network. As HTM professionals, we cannot simply ignore the IP and Subnet configurations of their medical devices. Our personnel are often asked to set the addresses on machines, and therefore it is best to have a firm grasp of network settings and their usages. Remember, if all else fails, there is an online subnet calculator and phone apps for this task. Use these tools to gain a better understanding of networks. The experience is irreplaceable.
Garrett Seeley, MS, CBET, Biomedical Equipment Support Specialist, VISN 17: VA North Texas Health Care System.
