1. DNS

2. 12.1 DNS resource records
12.2 The naming authority pointer (NAPTR) DNS RR
12.4 Service records (SRVs)

3. 12.1 DNS resource records Domain Name Service (DNS)
a distributed database holding the alphanumeric names and their corresponding IP addresses (and more) of every registered system on a Transmission Control Protocol (TCP)/IP network, such as the Internet or the IP Multimedia Subsystem (IMS)
each entry is referred to as a resource record (RR)

4. pc2 7.engineering.foobar.com.au
The alphanumeric names (domain names) are hierarchical in nature where country, company, department and even a host (machine) name can be identified
each step in the hierarchy is identified as a zone
The domain name below identifies a machine named pc27 in the engineering department of an Australian company called Foobar
pc2 7.engineering.foobar.com.au

5. An entry in a DNS server that maps a domain name to an IP address is referred to as an address record, or A record
In IPv6 these records are referred to as AAAA records

6. 12.2 The naming authority pointer (NAPTR) DNS RR
NAPTR RRs are used to replace compact, regular expressions with a replacement field that may well be a pointer to another rule
The DNS-type code for NAPTR is 35
Table 12.1 describes each field in detail
The format of a NAPTR record is as follows
Domain TTL Class Type Order Preference Flag Service Regexp Replacement

8. 12.2.1 NAPTR example This example uses the SIP service
A NAPTR RR looks like

9. In this example a client of the SIP service that does not support the Transport Layer Security (TLS) performs a NAPTR RR look-up for the "example.com." domain and selects the replacement
_sip._tcp.example.com

10. 12.4 Service records (SRVs)
An entity must know the exact address of a server to contact for a service
With the introduction of SRV RRs in [RFC2782], DNS clients can query a domain for a service or protocol and get back the names of servers
Administrators can use SRV RRs to distribute the load by using several servers to service a single domain
The DNS-type code for SRVs is 33
Table 12.2 describes each field in detail

12. _Service._Proto.Name TTL Class SRV Priority Weight Port Target
The format of an SRV record is as follows
_Service._Proto.Name TTL Class SRV Priority Weight Port Target

13. SRV example
Continuing with the example, the SRV RR looks like

14. In this example the client of the SIP service in the "examp
In this example the client of the SIP service in the "examp.com" domain performs an SRV look-up of "_sip._tcp.example.com" followed by an A record look-up for
server2.example.com