No, ARPA has nothing to do with an archaic Server Protocol used in the Internet stone age to translate a hostname to an Internet address.
ARPA stands for "Average Revenue Per Account," and it is one of these easy metrics. To calculate it, you will need to know your total yearly revenue and the total volume of your paid accounts.
Total Yearly Revenue / Total Number of Accounts = ARPA
Knowing your ARPA can help you forecast your revenue target and identify how much revenue your company generated during a given period.