Subnetting refers to the modification of an IP (Internet Protocol) network into smaller ,more logical IP networks that are known as subnets. The technique is employed to divide the address space of a unicast address prefix for allocation among the subnets of an organizational network. The prefixes are said to be “efficient” because they waste fewer possible addresses that are then made available for assignments to the subnets of an organizational network. Subnetting is necessary when a network number requires to be distributed across multiple local area network (LAN) segments. There are several situations when this becomes necessary, including;
* When an organization employs two different types of LAN technology, for instance, Ethernet and Token Ring
* When two network segments are limited by distance and remote offices need connectivity
* When segments need to be localized for network management purposes
* When there is a need to isolate hosts with the greatest bandwidth requirements
To illustrate this point, it’s important to consider the following. Subnetting using some class B address prefixes can support up to 60,000 nodes, which is far too many to have on one subnet. With subnetting, the space in that prefix address can be better utilized without requiring router reconfiguration. For instance, you can subnet 157.60.0.0/16 by using the first 8 host bits (called the third octet) for the newly created subnetted address prefix. If 157.60.0.0/16 is subnetted as explained, separate subnets with their own subnetted address prefixes can be created, and the number can go up to 254 host IDs on each subnet in this sequence; 157.60.1.0/24, 157.60.2.0/24, 157.60.3.0/24, 157.60.1.4/24, and so on. The router would detect the separated subnetted address prefixes and proceed to route the Pv4 packets to the appropriate subnet. After that, the subnetting of the prefix addresses as shown above will be undetectable to routers outside the network that is being subnetted. This is because the routers would still be regarding the nodes of the four subnets as being located on the address prefix 157.60.0.0/16.
Ideally, the decision to subnet should be taken before the workstations are allocated their IP addresses. Subnetting allows more flexibility in the segment layout and permits easier management capabilities. It also lends itself to faster troubleshooting. Another advantage of subnetting is that it keeps the size of Internet routing tables down, because there is no need to insert an additional number on each segment. Also, the routing announcement to the outside networks will be based on a single network number. However, subnetting may dramatically scale down the number of Internet Protocol (IP) addresses available to a network, and this may necessitate purchasing of new routers. Also, it’s important to keep in mind that some old routers do not send subnet masks for every announced route, as they do not understand the all 0’s and all 1’s subnets. This renders them unusable for subnet purposes. However, if a router allows for provision of netmasks in its routing table, the subnets in this case will be routable.
For more information on subnetting read this article.
