LAN Broadcast Domains
While switches help network performance by reducing collision domains, they will permit broadcasts to all users and multicasts to specific groups to pass through. In a switched network with a lot of broadcast traffic, network congestion can occur despite high speed backbones. As universities and colleges were often early adopters of Internet technologies, they may have large address allocations, perhaps even deployed as big flat networks which generate a lot of broadcast traffic. In some cases, these networks can be as large as a /16, and having the potential to put up to sixty five thousand hosts on a single network segment could be disastrous for performance.
The main purpose of subnetting is to help relieve network congestion caused by broadcast traffic. A successful subnetting plan is one where most of the network traffic will be isolated to the subnet in which it originated and broadcast domains are of a manageable size. This may be possible based on physical location, or it may be better to use VLANs. VLANs allow you to segment a LAN into different broadcast domains regardless of physical location. Users and devices on different floors or buildings have the ability to belong to the same LAN, since the segmentation is handled virtually and not via the physical layout.
References and Further Reading
RIPE Subnet Mask Information http://www.ripe.net/rs/ipv4/subnets.html
Priscilla Oppenheimer, Top-Down Network Design http://www.topdownbook.com/
– Main.AnnRHarding - 18 Jul 2005