The last editionof this column discussed subnet masks and the nature of IP addressing. In this and the following column, we will begin discussing what subnetting means and how to do it. First, though, we need to discuss some other basic stuff.

SUBNET

A network may be sub-divided into smaller units called “subnets.” As we will discuss, a subnet may be sub-divided into smaller subnets. There are specific rules for how to create subnets. 

NETWORKS, GATEWAYS & BROADCASTS

Each network and each subnet must have a network address and a broadcast address. These are always the smallest address in a network and the largest address in a network, respectively. Consider an array of IP addresses of 192.168.1.0 through 192.168.1.255, using the subnet mask 255.255.255.0. With the smallest address in the array being 192.168.1.0, this is the network address. The largest address in the array is 192.168.1.255, this is the broadcast address. The network address and the broadcast address may not be assigned to Hosts. Therefore, for any subnet, the number of available host addresses is equal to the number of addresses in the subnet, minus two. Gateways must be assigned among the remaining available addresses. It is common, but not required, for gateways to occupy the address adjacent to the network address. In this example, that would be 192.168.1.1. 

WHAT IS A DEFAULT GATEWAY? 

Imagine you are living in a large apartment building with several floors and many apartments on each floor. Apartments on the first floor might have numbers beginning with one, apartments on the second floor may have numbers beginning with two, and so on. Regardless, the apartment building has a street address that all the apartments share. For mail to get to your apartment, it first has to come to the street address; in an IP sense, the street address is the “gateway” to your apartment. A message from another apartment in the building can get to you without going outdoors. For a message from outside your building to get to you, it has to come through the building’s door. This is a bit like how a “default gateway” works—it’s the passageway in and out of your subnet. 

So let’s say the IP address of your computer is 10.24.24.120, with a subnet mask of 255.255.255.0. This means the first three octets of the IP address are the network. Your PC can only talk directly with other PCs in its network, so the default gateway needs to be in that network. Maybe the network admin has set it to be 10.24.24.1. This means that when you talk to computers outside your network, you are sending your communications first to 10.24.24.1, which then forwards them. 

IPv4 SUBNETTING BASICS

Before talking IPv4 subnetting, let me mention that there are two IP addressing schemes. IPv4 uses the dotted decimal notations we have been talking about, while IPv6 uses an entirely different system with enough address space (so I have heard) to count all the stars in the known universe with room left over. In these articles, we are talking only of IPv4. It is also worth mentioning that IPv6 will become more and more a part of everything as time goes by, because the public address space in IPv4 has been used up entirely. However, the private IPv4 address space is virtually infinite because these addresses cannot be used on the public internet. Therefore, they can be re-used over and over in any number of locations. 

Subnetting gives you the power to create (sub) network spaces. I like to use the analogy of a sheet of paper. Think of the surface of a sheet of paper as the addresses in a network. If you fold it in half, then unfold it, there will be a crease dividing the paper into two smaller parts. Now you have two sub-networks, with the original number of available addresses split between them. Take another sheet of paper and fold it twice. When you unfold it, you will have two creases that divide the paper’s surface into four parts. If you had made a third fold, you would have eight parts, and so on. Each fold doubles the number of parts, with the individual parts becoming smaller with each fold. If the paper is the network, each part is a subnet. Each fold reduces the number of addresses available for each subnet, but doubles the number of subnets. 

Suppose you have a request for seven subnets. If you fold the paper once, you have two subnets. If you fold twice, you have four. If you fold a third time, you have eight. Thus, three folds give you seven subnets with one left over. 

Next article, we’ll get a bit more into the meat of IPv4 basics. 

Tom Norman, CPBE is project engineer for Diversified.

For more background, read Parts I, II & III

For more background, read Parts I and II.

COLLISIONS AND BROADCASTS

If computers in an open network talk freely with one another and two computers start talking at the same time, you have a “data collision.” Collisions may be arbitrated via Carrier Sense Multiple Access/Collision Detection (CSMA/CD), which tells the computers on the network to stop talking and try again at random, usually avoiding a successive collision. With Ethernet switches, these collision domains are broken up because switches provide dedicated paths between hosts. Thus, other computers don’t see that traffic. An Ethernet switch breaks up collision domains. 

Nevertheless, each computer needs to announce itself to the network from time to time. It does this by broadcasting to all computers in the network. Ethernet switches do not block broadcasts. However, routers will not pass broadcasts from local hosts. Routers break up broadcast domains.

SUBNET MASKS

The subnet mask identifies which bits in an IP address belong to the network and which belong to the host. A subnet mask consists of a contiguous series of ones (1) followed by a contiguous series of zeros (0). It contains exactly 32 bits, broken up into four successive groups of eight bits (octets), in dotted decimal notation. It defines the network portion (designated by the contiguous string of ones), and the host portion (designated by the contiguous string of zeros) of the IP address. The default subnet masks for Class A, B and C IP addresses are: 

Table 8

Click on the Image to Enlarge

Examining an IP address of 192.168.2.10 with the default Class C subnet mask applied, the network address is 192.168.2, and the host address is 10. Let’s discuss how this works: 

Consider the IP address 192.168.2.10 and a Class C Subnet masks. Looking at one above the other, we get:

IP Address 11000000.10101000.00000010.00001010

Subnet Mask 11111111.11111111.11111111.00000000

The result is 11000000.10101000.00000010.00000000

This is the network address portion of the IP address. A subnet mask separates the network and host portions of an IP address.

CLASSFUL ADDRESSING

Before 1993, subnet masks were not used. The address class determined the size of the network and the subnet mask was implied but not used. The classes of addresses allowed for some networks to have a great many hosts, while others were smaller. This is called “classful addressing.” Though it still exists, you may never have to deal with classful addressing. Classful addressing means the subnet is fixed by the address class and cannot be altered.

CLASSLESS ADDRESSING

In 1993, Classless Inter-Domain Routing (CIDR) was introduced. IP address classes are ignored by assigning subnet masks. You might use Class A address space (10.x.x.x) and use the Class C subnet mask (255.255.255.0) to manage those addresses. This could create multiple networks in the Class A space, each with a limit of 255 addresses. Or suppose your IT department assigned you address space of 10.34.134.0/23. The /23 is an example of CIDR notation. The /23 says the subnet mask has 23 contiguous ones (1). That means the 23 contiguous ones (network) are followed by nine contiguous zeros (host). So what does this mean? First, let’s see what the subnet mask looks like in binary: 

11111111.11111111.11111110.00000000 shows a string of 23 ones. The first two octets each are 255. The third octet’s last bit is zero (0), so is value is 255 – 1 = 254. The subnet mask is 255.255.254.0. 

The third octet of the IP address is 134. We can figure out what this is in binary using a table: 

Table 9

First, we ask if 134 is bigger than 128. It is, so we set a 1 beneath the 128. 

Next, we subtract 128 from 134. The result is 6. The next number to the right not larger than 6 is 4. We set a 1 beneath the 4. 

Now, subtract that 4 from the 6 we had before, and we get 2. The next number to the right that is not larger than 2 is (surprise) 2. We set a 1 beneath the 2, and we have identified the binary ones (1) for the octet value 134. Set the remaining bits to zero (0) and we’re mapped to binary. 10000110 = 134. 

In this example, the subnet mask uses the first seven digits of the octet. The last digit is part of the host address space. With 9 bits available for the host address space, you have 512 host addresses including the network and broadcast addresses (more on this later). Your total address space includes the addresses from 10.34.134.0 through 10.34.135.255. 

Tom Norman, CPBE, is Project Engineer for Diversified.