
Mice and Men found that 38% of the .COM domains surveyed
had all their name servers on the same subnet.  And 75%
had one or more configuration errors.

http://www.menandmice.com/dnsplace/healthsurvey.html

DNS, like most databases, suffers from information entropy.

In other words, it takes a lot of energy to keep information
correctly updated while it is being changed.  Anyone who has
been Hostmaster for even a moderately sized ISP knows there
is an amazing number of ways for people to mess up any of the
pieces of data required to make the whole thing work.

As several people pointed out, you can't really assume close
IP addresses are in fact topologically close on the network.

For example, if you look at the name severs for GENUITY.NET

  Domain servers in listed order:

   DNSAUTH1.SYS.GTEI.NET	4.2.49.2
   DNSAUTH2.SYS.GTEI.NET	4.2.49.3
   DNSAUTH3.SYS.GTEI.NET	4.2.49.4

They appear to be closely related.  However, the addresses are
in fact routed to very diverse locations on Genuity's network.

You will find the same thing if you look at the name servers
for UU.NET

Domain servers in listed order:

   AUTH00.NS.UU.NET		198.6.1.65
   AUTH60.NS.UU.NET		198.6.1.181

These servers are also geographically diverse.

So I'm not sure if the 38% number is a true indication of how
much diversity DNS servers have.





