network number required any configuration, being obtained from a router. Each node dynamically chose its own node number, according to a protocol (originally the LocalTalk Link Access Protocol LLAP and later, for Ethernet/EtherTalk, the AppleTalk Address Resolution Protocol, AARP) which handled contention between different nodes accidentally choosing the same number. For socket numbers, a few well-known numbers were reserved for special purposes specific to the AppleTalk protocol itself. Apart from these, all application-level protocols were expected to use dynamically-assigned socket numbers at both the client and server end. Because of this dynamism, users could not be expected to access services by specifying their address. Instead, all services had names which, being chosen by humans, could be expected to be meaningful to users, and also could be sufficiently long to minimize the chance of conflicts. As NBP names translated to an address, which included a socket number as well as a node number, a name in AppleTalk mapped directly to a service being provided by a machine, which was entirely separate from the name of the machine itself. Thus, services could be moved to a different machine and, so long as they kept the same service name, there was no need for users to do anything different in order to continue accessing the service. And the same machine could host any number of instances of services of the same type, without any network connection conflicts. Contrast this with A records in the DNS, in which a name translates to a machine's address, not including the port number that might be providing a service. Thus, if people are accustomed to using a particular machine name to access a particular service, their access will break when the service is moved to a different machine. This can be mitigated somewhat by insistence on using CNAME records indicating service rather than actual machine names to refer to the service, but there is no way of guaranteeing that users will follow such a convention. Some newer protocols, such as Kerberos and Active Directory use DNS SRV records to identify services by name, which is much closer to the AppleTalk model. Protocols AppleTalk Address Resolution Protocol AARP resolves AppleTalk addresses to link layer, usually MAC, addresses. It is functionally equivalent to ARP and obtains address resolution by a method very similar to ARP. AARP is a fairly simple system. When powered on, an AppleTalk machine broadcasts an