Which network interface always exists in a Linux system?

The 'lo' loopback interface is automatically created by the Linux kernel during initialization and bound to the address 127.0.0.1. It does not depend on any physical hardware or additional configuration, making it the only interface universally present on all Linux systems. It is used for local inter-process communication and is essential for many system services.

sit0 is a virtual tunnel interface for IPv6-in-IPv4 encapsulation and is only present when that tunneling feature is loaded or configured.

wlan0 is a wireless LAN interface that only exists when a compatible wireless hardware adapter is present in the system.

vlan0 is a VLAN sub-interface that must be explicitly created and configured by an administrator; it does not exist by default.

eth0 is a wired Ethernet interface that depends on physical network hardware being present, and modern Linux distributions may use predictable naming (e.g., ens3, enp0s3) instead.