Tunneling

In Linux there are three types of tunneling (attaching network to network)
Mind that tunnels introduce headers overhead which must be accounted for when setting the link MTU.

IP in IP

IP packet encapsulated inside IP

GRE

Very similar to IPIP but supports multicast

Userland

Anything allowing to connect two networks that 'live' outside the kernel

References

1. https://tldp.org/HOWTO/pdf/Adv-Routing-HOWTO.pdf
2. https://developers.redhat.com/blog/2019/05/17/an-introduction-to-linux-virtual-interfaces-tunnels/