Editing AppleTalk (section)

=== AppleTalk Address Resolution Protocol ===
The AppleTalk Address Resolution Protocol (AARP) resolves AppleTalk addresses to [[link layer]] addresses.<ref name="cisco">{{Cite web |date=2010-02-02 |title=AppleTalk Overview |url=https://www.cisco.com/c/en/us/td/docs/ios/at/configuration/guide/12_2sx/atk_12_2sx_book/overview_appletalk.pdf |access-date=2023-03-21 |website=[[Cisco]]}}</ref> It is functionally equivalent to [[Address Resolution Protocol|ARP]] and obtains address resolution by a method very similar to ARP.<ref>{{Cite web |title=AppleTalk Address Resolution Protocol |url=https://www.devx.com/terms/appletalk-address-resolution-protocol/ |access-date=2025-01-29 |website=DevX |language=en-US}}</ref>

AARP is a fairly simple system. When powered on, an AppleTalk machine broadcasts an ''AARP probe packet'' asking for a network address, intending to hear back from controllers such as routers. If no address is provided, one is picked at random from the "base subnet", 0. It then broadcasts another packet saying "I am selecting this address", and then waits to see if anyone else on the network complains. If another machine has that address, the newly connecting machine will pick another address, and keep trying until it finds a free one.<ref name="macworld">{{Cite web |last=Faas |first=Ryan |date=2005-03-15 |title=Inside the Mac OS: A look at AppleTalk and zones |url=https://www.macworld.com/article/174889/appletalk.html |access-date=2023-03-21 |website=[[Macworld]]}}</ref> On a network with many machines it may take several tries before a free address is found, so for performance purposes the successful address is recorded in [[NVRAM]] and used as the default address in the future. This means that in most real-world setups where machines are added a few at a time, only one or two tries are needed before the address effectively becomes constant.