At some point, you may want to convert an IP address to a set of hostnames. This is known as a reverse lookup; it is implemented using PTR records in your DNS server. For example, say you have the IP address 22.214.171.124 and want to display its hostname(s) (apple.com). You have a few options:
Both of these functions accomplish the reverse lookup by having the resolver (lookupd on Tiger and earlier; DirectoryService on Leopard) hit up DNS for a PTR record matching the IP address. You can do the same from Terminal using dig -x 126.96.36.199. It’s a very long list.
So that works for registered DNS names. What about Bonjour (aka mDNS)? Well, Bonjour is just a distributed multicast DNS system. DirectoryService looks up entries in the local domain for you. This is why you don’t need to set up a DNS server but can still access all your machines with names like kitchen-imac.local. Bonjour also supports reverse lookups, but with one subtle caveat: your Mac wants to use IP
Try it for yourself; mDNS(1) is a debugging tool that lets you see what’s being advertised on your network. mDNS -B _daap._tcp . will show you all the shared iTunes libraries on your network. Then use mDNS -L “Your Music Library Name” _daap._tcp . to see which IP addresses are advertised over Bonjour.