Analyze Subnetting and Addressing Needs

Exam: 640-822 - Interconnecting Cisco Networking Devices Part 1


There are four basic areas that need to be addressed while analyzing the essentials for meeting addressing and subnetting requirements. The areas that need to be taken care of are:

  1. Grouping of hosts,
  2. The number of subnets required,
  3. The number hosts in every subnet,
  4. The size of the subnet.

Rules for Grouping

Every device connected to an IP internetwork requires having a IP address. The devices that are included are computers, laptops, servers, mobile devices and devices used for networking like firewalls, routers and switches. To put it in simple words any device that requires an IP for sending and receiving of packets requires an IP address. The assignment of the IP address follows two basic rules -

  • Addresses that belong to one subnet do not have the router as a separator
  • Addresses which do not belong to same subject do have one or more routers separating them

Hosting on the same subnets is based on the principle of having the same postal code for a particular region. The primary task of the routers is to forward the packets from a subnet to another. Routers are connected to multiple subnets.

PC A and B in One Subnet, PC C in a Different Subnet

PC A and B in One Subnet, PC C in a Different Subnet

Determining the Number of Subnets

The rules for determining the required number of subnets, are discussed in detail below. It is important that the network diagrams and configuration details of VLAN network engineer are available to the network engineer. The rules that determine the number of subnets are -

  • There should one subnet for every VLAN, Point-to-point serial link and Frame Relay PVC;
  • One subnet is required for every WAN link;
  • Current needs and scalability.

Determining the Number of Hosts per Subnet

For determining the number of hosts in every subnet it is important to have knowledge of simple concepts. Every device that is connected to a subnet requires an IP address. While determining the number of hosts for a new business it is important to pay attention to numbers of people and number of devices on the site. It is equally important to take into considerations the expansion plans. For an existing site the current levels along with the expansion plans are equally important. It is equally important to know the router interface IP address in every subnet and switch IP address used for remotely managing the switch. Planners usually have typical sites that are used as a basis for planning.

The Size of the Subnet

While determining the size of the subnet, it is crucial to determine if one size fits all or not. This is the final stage of initial planning. The size of the subnet also known as its length is determined by the number of IP addresses that are present in the subnet and are usable. A subnetting design can use one size subnet or subnets of different sizes. There are advantages and disadvantages of both.

Defining the Size of a Subnet

A subnet mask is assigned to each subnet. The mask defines the size of the subnet. The mask allocates host bits which perform the function of numbering the different host IP addresses that are present in the subnet.

  • One Size for All

    While deciding to use a single size subnet in a network, the same mask has to be used for all subnets as the size of the subnet is defined by the mask. The advantage of using a single size subnet is that the operations become simple. It is easier for everyone working on the network to find the subnet ID and the range of addresses and determine the number of hosts in a subnet.

  • Multiple Subnet Sizes (Variable Length Subnet Masks)

    For creating multiple subnet sizes in a single network, the engineer is required to use one mask for some subnets, while another mask for other subnets. Different masks is interpreted as different number of host bits.


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