iOS device can connect to iOS device via Bluetooth using:

• MultiPeer Local Multiplayer Plugin
• GameCenter Multiplayer plugin
• U3DXT

Android device can connect to Android device via Bluetooth using:

• Android Bluetooth Multiplayer (Pro)

https://github.com/DarkRay/Unity3D-bluetooth A low level bluetooth library.

Android and iOS cannot connect to each other via iOS as mentioned here.

Unity3D for iOS and Android: Multiplayer. Options for multiplayer game

• via Bluetooth (using Prime31 plugin)
  • iOS
• via LAN (using Unity RPC) The players can start combats in a Local Area Network with any of above devices: iOS vs iOS, iOS vs Android, Android vs Mac, and so on.
  • iOS
  • Android
  • Mac
  • Web (Kongregate)
• via Game Center (using Prime31 plugin): Uses the Game Center multiplayer to match combats.
  • iOS
• via Global Server (an in-house solution): The players can start combats around the world with any of above devices: iOS vs iOS, iOS vs Android, Android vs Mac, and so on.
  • iOS
  • Android
  • Mac
  • Web (Kongregate)

UNET the new Unity Networking library.

• Unet sample projects
• Multiplayer Networking Tutorial
• Unity 5 UNET Multiplayer Tutorials

TNET Another option which uses UDP for multiplayer game.

Alljoyn makes it easy for devices to communicate. Unfortunately it does not support Unity.