RFC3263推荐使用DNS，接续过程为NAPTR-> SRV -> A or AAAA，逐一尝试。
详见http://mit.edu/sip/sip.edu/dns.shtml，还提供了很好的实践。
关于Naming Authority Pointer (NAPTR) ：
This document describes a Domain Name System (DNS) resource record which specifies a regular expression based rewrite rule that, when applied to an existing string, will produce a new domain label or Uniform Resource Identifier (URI). Depending on the value of the flags field of the resource record, the resulting domain label or URI may be used in subsequent queries for the Naming Authority Pointer (NAPTR) resource records (to delegate the name lookup) or as the output of the entire process for which this system is used (a resolution server for URI resolution, a service URI for ENUM style e.164 number to URI mapping, etc). This allows the DNS to be used to lookup services for a wide variety of resource names (including URIs) which are not in domain name syntax. Reasons for doing this range from URN Resource Discovery Systems to moving out-of-date services to new domains. This document updates the portions of RFC 2168 specifically dealing with the definition of the NAPTR records and how other, non-URI specific applications, might use NAPTR. 其中ENUM - The bridge between the switched telephony network and the Internet 很是有用