Acknowledge:
Notification sent from one network device to another to acknowledge that some event (for example, receipt of a message) has occurred.
 

Application layer:
Totality of all applications and their relating protocols that use networks and have not yet been represented by the lower layers.
 

ARP:
Internet protocol used the Address Resolution Protocol to map an IP address to a MAC address. It is defined in RFC 826.
 

Arpanet:
Advanced Research Projects Agency Network. Landmark packet-switching network established in 1969. ARPANET was developed in the 1970s by BBN and funded by ARPA. It evolved into the Internet.
 

Arpanet Working Group:
blablabla
 

APNIC:
Organization that serves the Internet community in Asia by supplying user assistance, documentation, training, registration service for Internet domain names, and other services.

AS:
Autonomous System is a set of routers inside the domain administered by one authority.