execute programs. The only times this is usually done are when first assembling the system and also when upgrading or debugging the system. Certain high-security environments may require that a system be configured in this manner, but the utility of the system becomes limited when the system cannot be accessed over an external network. Institutions which decide to configure a Beowulf in this manner should include multiple front-end nodes, so that multiple users may simultaneously use the machine. When a system is configured in this manner, it is not necessary to pay any special attention to the IP addresses used. Any valid set of network addresses will do, but it is advisable to use the reserved address ranges mentioned in Chapter 5.
6.1.2 The Universally Accessible Machine
At the opposite end of the spectrum from the stand-alone configuration lies the universally accessible machine. In this configuration, each system node draws its IP address from the pool of addresses assigned to an organization. This allows internal nodes to be directly accessible by outside connection requests. In other words, every node is accessible from the entire Internet. The primary negative aspect of this configuration is that it greatly increases management tasks associated with security. It also unnecessarily consumes a large quantity of IP addresses that could otherwise be used by your organization's local area network. If your local area network already sits behind a firewall and uses a reserved address range, then this may be an appropriate configuration, allowing access to any node from any machine on your LAN. Also, some applications such as web and database load balancing servers, may require exposing all nodes to the external network. However, you will have to take care in arranging your network switches and associated connections so as to prevent LAN traffic congestion from interfering with Beowulf system traffic. In addition, if you choose to add multiple network interfaces to each node, you should probably not attach them to the external network.
6.1.3 The Guarded Beowulf
Somewhere in between the first two configurations stands the guarded Beowulf, which is probably the most commonly used configuration. The guarded Beowulf assigns reserved IP addresses to all of its internal nodes, even when using multiple networks. To communicate with the outside world, a single front-end, called a ''worldly node," is given an extra network interface with an IP address from the organization's local area network. Sometimes more than one worldly node is provided. But in all cases, to access the rest of the system, a user must first log in to
