The evolvability of a network architecture refers to its ability to be designed and implemented in a flexible and scalable manner, allowing it to adapt to changing needs and technological advancements. In MIN networks [1], network evolution can be realized through the extension of network identifiers. Specifically, various types of identifiers are employed to support different network protocols, and routers execute distinct operations on network packets based on the carried identifiers. This realizes the coexistence of various network protocols at the network layer. This chapter introduces the basic network identifier, identifier forms, identifier characteristics, identifier semantics, and identifier space in MIN networks.