This paper discusses the Constrained Application Protocol (CoAP), an application layer protocol introduced by Internet Engineering Task Force (IETF) Constrained RESTful environment (CoRE) Working Group for use with low power and resource constrained nodes in the internet of things. The paper discusses the CoAP architecture supported with suitable examples. CoAP protocol message format, types, methods and security feature is also discussed. In this paper, we present a CoAP based car parking system implemented using Contiki, an open source operating system for resource constrained device. The Cooper (Cu) CoAP user agent for Firefox browser is used for handling CoAP URI scheme.