Understanding Subnetting

Understanding IP addressing and Subnettting

An IP address can is a unique numeric identifier that is assigned to each computer in a TCP/IP based network. To communicate on the Internet and private TCP/IP network, all hosts defined on the network must have IP addresses. These 32-bit IP address identifies a particular host on the network.

An IP addresses consists of the following components:

• Network address or ID: This identifier uniquely identifies a specific network. All computers and all other devices on the same network, have the identical network address as a segment of their IP address.

• Host address or ID: Each TCP/IP host on the network has a uniquely assigned host address (host ID). The host address has to be unique so that each machine on the network can be uniquely identified. Hosts are devices such as servers, routers or workstations.

An IP address can be represented in the following notations:

• In the dotted-decimal IP address notation, the digits 0 to 9 are used. With the 32-bit IP addresses, the octets and bit places are numbered from the left to the right. Each number in the decimal system can be represented in the binary format.

• The binary notation uses only two digits, which are 0 and 1. A binary digit is called a bit.

With IP addressing, an octet is formed by 8 bits. An IP address therefore consists of 32 bits, or four octets. The first octet refers to the leftmost octet. The bit places 1 to 8 signify the eight leftmost bit places. The second octet signifies the following eight bits, which is bit places 9 to16. The third octet signifies the following eight bits, which is bit places 17 to 24. The fourth octet signifies the following eight bits, which is bit places 25 to 32.

The IP Address classes are:

• Class A addresses were created for tremendously large networks with many hosts. With Class A addresses, the first high-order bit is zero. The first 8 bits or octet is the network address. The second octet, third octet and fourth octet defines the host address or ID. The maximum number of Class A networks that can be created is 128. The figure of possible host addresses is 16,777,216.

• Class B addresses were created for networks that are medium of size and which have an average number of hosts. With a Class B address, the initial two high-order bits are set to 10. The first 2 octets define the network address. The third octet and fourth octet defines the host address or ID. The maximum number of Class B networks that can be created is 16,384. Each Class B network can have 65,534 possible host IDs.

• Class C addresses were created for networks that are small and that have few hosts. With a Class C address, the initial three high-order bits are set to 110. The first 3 octets define the network address. The fourth octet defines the host address or ID. The maximum number of Class C networks that can be created is 2,097,152. Each Class C network can have 254 possible host IDs.

When an organization has numerous computers, or it has computers that are geographically dispersed, it would be a good idea to divide a larger network into a number of smaller networks that are connected via routers. These smaller logical networks are called subnets. Subnetting is considered a vital component of IP addressing.

The advantages associated with subnettng a network are summarized below:

• Through subnetting, you can reduce network traffic and thereby improve network performance. You only allow traffic that should move to another network (subnet) to pass through the router and to the other subnet.

• Subnettiing can be used to restrict broadcast traffic on the network.

• Subnetting facilitates simplified management. You can delegate control of subnets to other administrators.

• Troubleshooting network issues is also simpler when dealing with subnets than it is in one large network.

Implementing Subnetting

The Important factors that should be clarified when determining the requirements of your subnetting scheme are:

• The number of required network IDs. A network ID is needed for each subnet, and for each WAN connection.

• The number of required host IDs. A host ID is needed for each TCP/IP based network device

Using the information above, you can create

• A subnet mask for the network.

• A subnet ID for every physical network segment

• A range of host IDs for every unique subnet

You implement subnetting by assigning a subnet address to each machine on a particular physical network. While you cannot change the network address segment of an IP address, you can change the host address segment. With subnetting, you take part of the host address and reuse it as a subnet address. This is done by taking bit positions from the host ID and then changing it to the subnet identifier. The number of host IDs are therefore reduced when you implement subnetting.

When you start the subnetting process, the bit position taken from the host ID reduces the number of hosts by a factor of 2. For instance, in a Class B network, you can have 65,534 possible host addresses or IDs. If you start subnetting the number of hosts which you can have is about half that figure. This is calculated as 65,534 / 2.

If the network has been subnetted, you can use the following equation to determine the number of host IDs you can have for each subnet:

• 
  2x - 2

  • 
    x = number of bits in the host ID