| | | | | | a worldly node. The benefit of this approach is that you don't consume precious organizational IP addresses and you constrain system access to a limited number of controllable access points, facilitating overall system management and security policy implementation. The disadvantage is that it is not possible for internal nodes to access the external network. But that can be remedied by using IP masquerading which will be discussed later. For increased security, it is often desirable to place the worldly nodes behind a firewall. In the rest of this chapter, we will use the Guarded Beowulf as the canonical example system unless explicitly stated otherwise. | | | | | | | | | Beowulf system components need to communicate with each other, and for intercomponent communication to be possible, each component, or node, requires a unique name. For the purposes of this chapter, node naming refers to both the assignment of IP addresses and hostnames. Naming workstations on a LAN can often be quite arbitrary, except that sometimes segmentation of the network restricts the IP addresses available to a set of workstations located on a particular segment. Naming Beowulf nodes requires a bit more thought and care. | | | | | | | | | Beowulf clusters communicate internally over one or more private system area networks. One (or perhaps more, for redundancy and performance) of the nodes has an additional network connection to the outside. These nodes are referred to as worldly nodes to distinguish them from internal nodes, which are connected only to the private cluster network. Because the internal nodes are not directly connected to the outside, they can use the reserved IP addresses discussed in Chapter 5. Specifically, most clusters assign their worldly node to address 192.168.1.1, and assign internal nodes sequentially to addresses in the range 192.168.1.2 to 192.168.1.253. The worldly node will always have a second network interface, possessing a routeable IP address that provides connectivity to the organizational LAN. | | | | | | | | | From long experience, we have found that internal hostnames should be trivial. Most Beowulf clusters have assigned very simple internal hostnames of the format <cluster-letter><node-number>. For instance the first Beowulf named its nodes using simply the letter b as a prefix, but made the mistake of calling its first node bO. While it is natural for those steeped in the long-standing computer science tradition of starting indices at zero, it is better to map the numbers contained in hostnames directly to the last octet of the node IP address. For example, 198.168.1.1 becomes node b1, 198.168.1.2 becomes b2, etc. As you can see, there can be no bO node, because 198.168.1.0 is a network number, and not a host address. | | | | |
