Apple moved the Network Utility in Mavericks. Instead of being located in /Applications/Utilities, like it has been since OS X 10.3, they have moved Network Utility to /System/Library/CoreServices/Applications.

Some people have reported that the Network Utility is removed from their Mac after upgrading to Mavericks, even from the /System/Library/CoreServices/Applications folder where it’s supposed to be. If this occurs, it can be copied from another Mac running OS X 10.8 or 10.9.