We pick k.root-servers.net, one of the 13 root servers (1), and ask it about vapor.isi.edu and learn that edu is delegated to six name servers (2), so we ask d.edu-servers.net where we learn that isi.edu is delegated to four name servers, including ns.east.isi.edu (3), so we ask that one and get the following answer:
The four mail servers boreas, nitro, vapor and yin.isi.edu together
There are one domains that use the four mail servers boreas, nitro, vapor and yin.isi.edu together, hereafter referred to as "mail server group 1".
All of those use the four name servers nitro.isi.edu, ns.isi.edu, vapor.isi.edu and ns.east.isi.edu together, hereafter referred to as "name server group 1". Worth noting is that all of the domains that use name server group 1 use mail server group 1.
All of them point only to the IP number 128.9.176.20.
The four name servers nitro.isi.edu, ns.isi.edu, vapor.isi.edu and ns.east.isi.edu together
There are one domains that use name server group 1.
All of those use mail server group 1. Worth noting is that all of the domains that use mail server group 1 use name server group 1.
All of them point only to the IP number 128.9.176.20.
The mail server vapor.isi.edu
There are three domains that use the mail server vapor.isi.edu.
One of those use the two mail servers darkstar and vapor.isi.edu together, hereafter referred to as "mail server group 2".
One of them point only to the IP number 128.9.128.127.
One of them use name server group 1.
One of them point only to the IP number 128.9.64.64.