There also is a range of possible interconnect networks. The most commonly used networks are Ethernet, fast Ethernet, and Myricom. Myricom is the fastest, but it is very expensive. Fast Ethernet gives the best performance for the money. Fast Ethernet is typically used in a star configuration with a hub or switch at the center.
Switches are more expensive than hubs, but are important for reasonable network performance.
The head is attached to both the private network of the cluster and the external network. The other nodes are not directly connected to the external network. All of the software needed to set up a Beowulf cluster is free, and anyone may modify and redistribute the source code as well. The software is developed and maintained by volunteers all over the world. One of the most impressive clusters to date is the Avalon cluster built by the Center for Nonlinear Studies and the Theoretical Division of Los Alamos National Lab.