The EPICS Channel Access PV Name Server is both a Channel Access server and Channel Access client. It provides a means for clients to find process variables without the IOCs having to receive and answer all the search requests from those clients. It can also be used to connect multiple network subnets together.
The Name Server may run on a machine with multiple network cards, so the clients and servers may be on different subnets.
More details are available on the Nameserver webpage at APS.