A data processing system comprising a host computer system and a network interface device for connection to a network, the host computer system and network interface device being coupled together by means of a data bus, and: the network interface device comprising: a controller unit having a first data port for connection to a network, a second data port, and a data bus interface connected to said data bus, the controller unit being operable to perform, in dependence on the network endpoints to which data packets received at the network interface device are directed, switching of data packets between the first and second data ports and the data bus interface; and an accelerator module having a first medium access controller coupled to said second data port of the controller unit and a processor operable to perform one or more functions in hardware on data packets received at the accelerator module, the said first medium access controller being operable to support one or more first network endpoints; the host computer system supporting: a plurality of guest software domains including a first guest software domain having an application; and a privileged software domain configured to present a virtual operating platform to said plurality of guest domains, the privileged software entity including a first software driver for the controller unit but not including a second software driver for the accelerator module; wherein the application is configured to access the accelerator module by means of one or more first data packets formed in accordance with a predetermined network protocol and directed to one or more of the first network endpoints.