Multipath TCP is a new TCP extension that attracts a growing interest from both researchers and industry. It enables hosts to send data over several interfaces or paths and has use cases on smartphones, datacenters or dual-stack hosts. We provide the first analysis of the operation of Multipath TCP on a public Internet server based on a one-week long packet trace. We analyse the main new features of Multipath TCP, namely the utilisation of subflows, the address advertisement mechanism, the data transfers and the reinjections and the connection release mechanisms. Our results confirm that Multipath TCP operates correctly over the real Internet, despite the presence of middleboxes and that it is used over very heterogeneous paths.