dns2.rb.net.au has one IP number , which also has a corresponding reverse pointer. mrug.net, auweb.com, rb.net.au, inana.net, wc.com.au and at least 100 other hosts use this as a nameserver. rb.net.au is a domain controlled by two nameservers at rb.net.au themselves. They are on different IP networks. Incoming mail for rb.net.au is handled by two mailservers also at rb.net.au. They are on different IP networks. rb.net.au has one IP number. dns2.rb.net.au is hosted on a server in Australia.