Which network interface always exists in a Linux system?

The 'lo' loopback interface is a software-only virtual interface instantiated by the Linux kernel itself during boot on every system regardless of physical hardware; it provides the 127.0.0.1 address used for local inter-process communication and requires no network adapter, driver, or configuration to exist.

The 'sit0' interface is an IPv6-in-IPv4 tunnel device that is only present when the 'sit' kernel module is loaded, which does not happen on every Linux system by default.

The 'wlan0' interface only exists when a compatible wireless network adapter is present in the system and its driver is loaded, so it is absent on systems without wireless hardware.

The 'vlan0' interface is created only when VLAN tagging is explicitly configured by an administrator and is not automatically present on any standard Linux installation.

The 'eth0' interface depends on the presence of a compatible wired Ethernet adapter, and on modern systems using predictable network interface naming the device may be named 'ens3', 'enp2s0', or similar rather than 'eth0'.