Simple definition
IP-in-IP encapsulates one IP packet inside another to create a network tunnel without providing encryption by itself.
Purpose and use
IP-in-IP encapsulates an IP packet inside another IP packet to carry a network across intermediate infrastructure.
How does it work?
IPv4-in-IPv4 uses outer IP protocol number 4, without a dedicated TCP or UDP tunnel port.
Example and verification
Verify that each endpoint can reach the other's outer address and that inner routes do not recursively send the tunnel into itself.
Points to watch
The tunnel does not encrypt data and reduces available payload size through its additional header.
In short
Encapsulation and routing provide connectivity; filtering, MTU and cryptographic protection remain separate design choices.