Planet Networking & Communication CS-2001 User Manual page 693

 Supposing a user wants to send an email to mary@mail.nu.net.tw. The user
is using test.com.tw as its SMTP server. The DNS records will be queried
on this server to determine where to send the email destined for
mail.nu.net.tw. The following table shows the MX record resulted from the
query:
（Table 24-4）
Domain Name
host3.nu.net.tw
mail.nu.net.tw
 It can be inferred from the table above that emails will be
transferred to host3.nu.net.tw via the SMTP protocol.

Sender Policy Framework(SPF) : SPF, serves as an authentication mechanism,
rejects forged addresses in the SMTP MAIL FROM (Return-Path). It uses DNS
TXT records to identify the identity of a sender, efficiently preventing spam
emails and phishing.
 MX shoulders the responsibility for directing emails depending on the DNS
record. If a mail server is required to be replaced, then simply by changing
the DNS MX record will allow the mail service to remain in operation.
When an email is received from a different domain, the mail server will
check if the sender's mail server IP is on the SPF list or not.

(AAAA) IPv6 : Like IPv4 A(Address), (AAAA) IPv6 is used for mapping up IP
addresses and domain names.
 IPv6 is designed to succeed IPv4. It has a larger address space than IPv4.
IPv6 uses 128 bits whereas IPv4 uses only 32 bits.
Domain Name
host33.nu.net.tw
Table 24-5 Domain Name and IP Address Mapping Table
Reverse

It reverses a DNS lookup by using its IP address. DNS lookup is distinguished
into forward DNS lookup and reverse DNS lookup. Forward DNS lookup serves
as a translator to transform a hostname address (e.g. www.yahoo.com) into a IP
address. On the other hand, reverse DNS lookup does the reverse.

For IPv4, the process of reverse resolving an IP address uses the
pointer DNS Record type(PTR record). For example, an address(A)
record for host1.nu.net.tw points to the IP address 61.11.11.12. In
Table 24-4 MX Record of Querying mail.nu.net.tw
Type
AAAA
Type IP Address
61.11.11.10
A
host3.nu.net.tw
MX
（Table 24-5）
IP Address
FEC0::2AA:FF:FE3F:2A1C
692