mes the size of the 32-bit IPv4 address stored in DNS A resource record type. Figure 3.24 illustrates how both AAAA and IPv6 type of Pointer Record (PTR) records can be con- figured on a DNS server by using Internet Systems Consortiums' BIND-style format. The name BIND comes from Berkeley Internet Name Domain - see for more information at http://www.isc.org. The increased length of IPv6 addresses when compared to IPv4 results in larger DNS response messages. The maximum length of DNS protocol payload is 512 bytes over UDP transport [141], which can become a limiting factor and hence result in truncated messages. Therefore, it is highly recommended that implementations support 'Extension Mechanism for DNS (EDNSO)' [143], which enables requestors to inform recursive DNS servers about the capability to receive larger responses. The maximum theoretical size for DNS messages is increased to 64KB by the ENDSO, but due to fragmentation and Path MTU issues it is recommended that requestors advertise capability to receive packets with AAAA record: www.example.com IN AAAA 2001:db8:0:0:0:0:1234:abcd PTR record: $ORIGIN 0.0.0.0.8.b.d.0.1.0.0.2.IP6.ARPA. 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0 IN PTR ns1.example.com. 2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0 IN PTR mail.example.com. Figure 3.24 BIND-style examples of AAAA and PTR type of DNS resource records. Deploying IPv6 in 3GPP Networks size of 1280 bytes, or more if Path MTU discovery between DNS requestor and server indicates the network capability to transport larger packets. A6 Record Type While today AAAA records are dominant, there was a competing record type called A6. The A6 was originally defined in RFC 2874 [144], but was eventually declared historic by RFC 6563 [145], which discusses in depth what were the reasons for A6 record type's failure. These included, for example, the general problem of having two tools for the same thing - either AAAA or A6 had to go - slower latency to resolve A6 queries, and increased security risks when compared to AAAA. Forward DNS Q