Definition of a Subnet

The specification defines a subnet as follows: a set of ports and associated links with a common Subnet ID and managed by a common Subnet Manager (SM).

The SM is the entity that discovers all of the devices in the subnet at startup time, configures them, and then performs a periodic sweep of the subnet to detect any changes to the subnet's topology.

Refer to Figure 1-3 on page 13. In the illustration, all of the CA and router ports, as well as switch management ports (switch port 0—typically an internal port with no connector for a physical link to attach to) that can exchange packets solely by traversing switches (not routers) are said to reside in the same subnet. During configuration, the SM assigns each of these ports a unique Local ID address (referred to as the LID address) as well as a common Subnet ID (also referred to as the Subnet Prefix) that identifies the subnet that a port resides in. Subnets may be connected to each other through routers.