The Network Utility has been moved in OSX 10.9 Mavericks, it used to be in the Applications/Utilities folder but now has been moved from there into the Core Services directory.

Get it with Spotlight

Just get to it with Spotlight.

The actual Core Services directory can be found in /System/Library/Core Services and Network Utility is one more folder deep – /System/Library/Core Services/Applications.

Seems more Utility based apps are being migrated into this spot, all the functions in Network Utility can also be performed in Terminal.