For those who may not be familiar with Brentwood:
Originally known as Modern Times, Brentwood is a hamlet (an unincorporated area) in the northwest section of the Town of Islip, in the southwest part of Suffolk County, along the Islip/Smithtown, Islip/Babylon and Islip/Huntington town lines.
Beginning on the north and moving in a clockwise direction, the Hamlet of Brentwood in the Town of Islip is bordered on the north by Hamlet of Commack and the Hamlet of Hauppauge in the Town of Smithtown (the Islip/Smithtown town line); on the east by the Hamlet of Central Islip and the Hamlet of Islip; on the south by the Hamlet of Islip, the Hamlet of North Bay Shore and the Hamlet of Baywood; and, on the west by the Hamlet of Deer Park in the Town of Babylon (the Islip/Babylon town line) and the Hamlet of Dix Hills in the Town of Huntington (the Islip/Huntington town line).
The Hamlet of Brentwood has a different border than does the "Brentwood, NY 11717" postal zone (i.e., a place can have a "Brentwood, NY 11717" mailing address and not be in the Hamlet of Brentwood): places that have a "Brentwood, NY 11717" mailing address that are not in the Hamlet of Brentwood are in the Hamlet of Dix Hills in the Town of Huntington.
For a good set of town-by-town maps showing all the villages and hamlets in each of LI's 13 towns (3 in Nassau County and 10 in Suffolk County):
Excellent Long Island Geographic Resource