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.
However the 4/8 route is what is advertised to the world, and there are certainly ocassions where that route fails to be propagated. It's more diverse than adjacent nodes on an ethernet, but hardly as diverse as would be ideal. Ideally, all DNS servers for a site shouldn't be in the same autonomous system. --jhawk (who recently made the observation that there are VBNS-connected root nameservers, but not VBNS-connected gtld servers, so a hypotehtical site with a VBNS connection and a commodity connection has great difficulty using their VBNS connection to resolve VBNS names when the commodity connection goes down)