A: At its most basic, Citrix XenServer communicates mostly over TCP port 443. This port is used for pool synchronization as well as connectivity to XenServer's Web Self Service and vSwitch Controller virtual appliances. TCP port 22 is used for any Secure Shell (SSH) communication that occurs either between servers or via administrator management connections. Two ports, TCP 27000 and TCP 7279, connect the Citrix License Manager and its vendor daemon.
Although these ports serve core functions, they aren't XenServer's only ports. Storage Link connections require TCP port 21605 for SOAP over HTTP. iSCSI storage connections use a default TCP port of 3260, NFS storage uses TCP port 2049, and CIFS storage requires TCP ports 139 and 445. The range of ports grows even wider when XenServers are deployed by using Citrix Provisioning Services.
One of the nicest visualizations of XenServer's port and connection requirements is a large-format poster put together by Björn Andersson, which you can download from his blog. The poster does an excellent job of showing the array of interconnections between a XenServer and all of its accompanying infrastructure.