Measures for controlling communication access in a data center network are provided. A packet forwarding function in a server in a data center network is configured to access an access control data store when making forwarding decisions for received data packets which are being routed to/from virtual systems hosted on that server. In response to receipt, at the server, of a setup notification relating to setup of a virtual machine on the server, one or more entries are populated in the access control data store. The entries comprise an internet protocol (IP) address of the virtual system and at least one associated IP address of one or more other communication endpoints.