Since it’s release, Yosemite has been plagued with various networking issues, including frequent instability and/or dropped connections with both wired and WiFi networks. Previous Apple patches have helped some, but not all.
Software developers have been blaming these networking issues on Yosemite’s new discoveryd process, which replaced the mDNSResponder process Apple has used to help manage the network stack since OS X 10.2.
The OS X 10.10.4 update removes the discoveryd process, and replaces it with the tried and true mDNSResponder. So far results posted to Apple’s discussion boards have been positive, but because this is a major rollback it could affect any software or process that sends calls directly to mDNSResponder. I’ve yet to hear of this causing a problem, but keep this in mind when troubleshooting odd network or DNS issues on Macs recently updated to OS X 10.10.4.