If we use a DNS lookup tool to query the DNS server for the network in our example (for all information, in verbose mode), you would see entries like the following:
domain.com. IN MX 50 cecil.domain.com.
IN MX 10 mail.domain.com
cecil.domain.com. IN A 156.21.50.100
mail.domain.com. IN A 156.21.50.5
5.50.21.156.in-addr.arpa.,type = PTR
host = mail.domain.com
5.100.21.156.in-addr.arpa.,type = PTR
host = cecil.domain.com