Most computer systems have names: axiom, amino,
legio, or compute-0-4. When you want to interact with these
particular systems you contact them by name. You may notice though that not all
names work at all places. Like legio may know axiom and
amino by name, but axiom and amino do not know
legio and are unable to establish a connection back to that
workstation.
It is important to realize that not all systems know the names for other
systems (or may know them by different names). Conceptually, you should think
of each system as having the computer equivalent of a personal address book, in
it are the names and numbers of all the systems that it needs to talk to. Once
it calls a system (using ssh or scp), its
like any phone conversation and information can be passed back and forth on
that phone line until the connection is broken. Unfortunately, not all systems
have everyone in their address books. like in real life, if an address
book is too large, it can take a long time (relatively) to find what the system
is looking for. This leads to situations where legio might have
axioms number, but axiom may not have
legios number.
Practically, this concept is important to understand when you want to copy
data from the cluster to another location: a workstation, a backup system, or
another cluster. system may not know the name of another system on a
first name basis. So when you try ``scp data.file
legio:~'' on axiom, axiom will return an
error saying it cant find legio. In these cases,
most often you need to look at a phone book (in computer terms - DNS) and to
find someone in the phone book you need their full name. As briefly shown
above, the full name of ``axiom''
is ``axiom.ccmb.med.umich.edu''. So
if a system doesnt know axiom by first name, it may
have more luck with its full name. Its also important to
remember, just like in real life, most computer phone books (DNS) are for local
areas. So while it may have all the entries for everything in its area, and be
able to look up most internet names, there may not be a listing for a system
located in a another area. In real life a phone book for Washtenaw county
doesnt have a listing of all the telephone numbers of people
in Wayne county, even though the two areas are neighbors.