Network Subnetting: Divide your network

Subnetwork is a logical division of a local area network, which is created to improve performance and provide security. It describes networked computers and devices that have a common, designated IP address routing prefix. Subnetting is used to break the network into smaller more efficient subnets to prevent excessive rates of Ethernet packet collision in a large network. To enhance performance, subnets limit the number of nodes that compete for available bandwidth and such subnets can be arranged hierarchically, into a tree-like structure. Routers are used to manage traffic and constitute borders between subnets. In an IP network, the subnet is identified by a subnet mask, which is a binary pattern that is stored in the client machine, server or router.

The advantages of Subnetting a network are:

  • Through subnetting, user can reduce network traffic and thereby improve network performance. User only allows traffic that should move to another network (subnet) to pass through the router and to the other subnet.
  • It can be used to restrict broadcast traffic on the network.
  • It facilitates simplified management. User 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

For implementing the subnetting scheme, the user should keep the important factors clarified while determining the requirements like, the number of required network IDs, which is needed for each subnet, and for each WAN connection and the number of required host IDs, which is needed for each TCP/IP based network device.

Using the information above, user can create a subnet mask for the network, a subnet ID for every physical network segment and a range of host IDs for every unique subnet.

In the process of subnetting, the bit position taken from the host ID reduces the number of hosts by a factor of 2. For example, in a Class B network, you can have 65,534 possible host addresses or IDs. So, if you start subnetting the number of hosts which will be about half that figure, i.e., 65,534 / 2.

Network address and logical address

The term network address either refers to logical address, i.e. network layer address such as the IP address, or to the first address (the base address) of a class-full address range to an organization. PCs and devices, which are part of an internetworking network for e.g. Internet, each have a logical address.

The network address is unique to each device and can either be dynamically or statically configured. An address allows a device to communicate with other devices connected to a network. The most common network addressing scheme is IPv4.

An IPv4 address consists of a 32 bit address written, for human readability, into 4 octets and a subnet mask of like size and notation. In order to facilitate the routing process the address is divided into two pieces viz. the network prefix that is significant for routing decisions at that particular topological point and the network host that specify a particular device in the network.

The primary reason for subnetting in IPv4 was to improve efficiency in the utilization of the relatively small address space available, particularly to enterprises.

Subnetting in IPv6 networks

Subnetting is also used in IPv6 networks. But, in IPv6 the address space available even to end-users is so large that address space restrictions no longer exist.

In IPv6, the recommended allocation for a site is an address space comprising 80 address bits and for residential customer network, it may be as small as 72 bits. This provides 65,536 subnets for a site, and a minimum of 256 subnets for the residential size.

An IPv6 subnet always has a /64 prefix which provides 64 bits for the host portion of an address. Although it is technically possible to use smaller subnets, but they are impractical for LAN as stateless address auto-configuration of network interfaces (RFC 4862) requires a /64 address.

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