If we detect that you have more than one name server in the same class C subnet (the first three numbers of their IP are the same), we will issue this warning.
Having more than one (1) name server in the same class C subnet is not recommended, as this increases the likelihood of a single failure disabling all of your name servers.
Additional Information
If your servers are actually dispersed or you have other means in place to generate redundancy in your DNS, you can feel free to filter this warning with our Custom Filters settings.
RFC 2182:
Secondary servers must be placed at both topologically and
geographically dispersed locations on the Internet, to minimise the
likelihood of a single failure disabling all of them.
That is, secondary servers should be at geographically distant
locations, so it is unlikely that events like power loss, etc, will
disrupt all of them simultaneously. They should also be connected to
the net via quite diverse paths. This means that the failure of any
one link, or of routing within some segment of the network (such as a
service provider) will not make all of the servers unreachable.
[RFC 2182]
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