__Subnetting__

Like it or not, you’ll need to understand subnetting and TCP/IP principals in order to be successful on the 70-240 exam. For all intents and purposes, TCP/IP is considered to be assumed knowledge based on its importance as a protocol. While you might not get many questions relating to subnetting on the Professional portion of the exam, I have decided to cover the topic earlier rather than later in order to ensure that people have plenty of time to practice and prepare. The purpose of this article in not to teach you how to create a subnetting scheme for a large network. Instead, my purpose here is to help you recognize problems with the IP configuration of a host. I’ll teach you how to figure out which host IDs are valid on a subnet, what the mask really means, and how to calculate valid ranges of addresses on a network. Once you can do these things, you can handle any question that might be thrown at you.

It sometimes amazes me that people get so worked up about subnetting, because it really is quite simple. First of all, you need to recognize that in order to really understand subnetting (at least starting off), looking at the numbers in decimal notation makes very little sense. You need to be looking at numbers in binary to really understand what is happening. The beauty of binary numbering is its simplicity – each value can only be a 1 or a 0. Note that each section (octet) of an IP address can be represented by a series of eight bits. There are 4 octets, so 32 bits altogether. That means any IP address can be also looked at as a 32-bit binary number. The table below outlines binary numbering corresponding values.

Decimal | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |

Binary | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |

What this means is simple. If I were to ask for the value of 11001100 in decimal, it would be 128+64+0+0+8+4+0+0, which equals 204. Each bit corresponds to the decimal value above it – add the values for each ‘1’ value and you have the answer. 11111111 would be 128+64+32+16+8+4+2+1, which equals 255 (which is also the highest possible decimal value in an 8-bit binary number).

But what about converting decimal numbers to binary? Well, it’s different, but no more difficult. Start at the left on the chart above, and add the decimal values together until you reach your total. Every number you use is a ‘1’ and every number you leave out is a ‘0’. For example, let’s take the number 77. This would be 01001101. Say what? Well, I just started adding numbers left to right, leaving out numbers that put me over 77. In this example, I have 0+64+0+0+8+4+0+1. Simple.

You can also do this using a calculator program with a scientific mode. Just type is a number in decimal and hit the BIN button. The number will then be displayed in binary. However, the calculator has no idea that you’re dealing in 8-bit numbers, so you’ll have to be careful. For example, my calculator will tell me that 77 in binary is 1001101. That is, it leaves off any leading zeros. As such, you’ll need to remember to ‘pad out’ your binary numbers to 8 bits if you use the calculator. For example, the calculator will show decimal 8 as binary 1000. For an IP address, we need to add the 4 other zeros, making it 00001000. You’ll have access to the calculator on the exam, so know how to use it.

After you understand binary numbering, subnetting is easy. First of all, we need to discuss what subnetting is. Quite simply, it is taking a big network ID and breaking it down into a number of smaller networks, or subnets. Routers are what usually separate subnets. Reasons for subnetting include connecting different topologies (such as Ethernet and Token ring), as well as making networks smaller and more manageable. Subnets are also sometimes referred to as broadcast domains, since a broadcast sent on a subnet goes to all hosts on that subnet

For the purpose of the 70-240 exam, you will need to recognize and understand how subnetting works. This includes being able to view system configurations and determine why clients are having trouble communicating. As such, you’ll need to be able to recognize valid IP addresses, subnet mask values, and what range of IP addresses are valid on a given subnet. Let’s start with a look at valid subnet mask values.

A subnet mask means little in decimal. In binary, however, they tell a story. The subnet mask is what tells us which of the 32-bits in an IP address represent the network identification, and which represent the host identification. In the example below, the host IP address is 156.77.11.3 and the subnet mask is /21, or 255.255.248.0. In decimal, it is difficult to determine which portion represents the network and which the host. However, it binary the mask value is:

11111111 11111111 11111000 00000000

So what does that tell me? That the first 21 bits are used to represent the network, and the last 11 bits are used to represent a host on the network. Actually, it tells me more than that. It also tells me how many hosts I can have per network. How? Well, if eleven bits are used to represent a host, then this subnet can have 2046 hosts. How did I get that? Simple:

2 to the power of 11, minus 2. That equals 2048 minus 2, or 2046. Why minus 2? You subtract 2 because a host value of all binary 0’s represents the subnet, and a value of all binary 1’s is the broadcast address for this subnet.

If the subnet mask in the example above had been /17, or 255.255.128.0, that would leave 15 bits for host addresses. That would mean 2 to the power of 15 minus 2 hosts, or 32766 total.

Figuring that stuff out should now be easy enough as well. The big question, and the key thing you need to be able to do, is to be able to determine if a host ID is valid on a subnet. Every subnet has a range of addresses that are valid on it. In my last example, there were 32766 valid host addresses. You need to be able to determine which ones are valid for the subnet. It isn’t that hard, but you need to know what you’re looking for.