IP aliasing
Assign multiple IP addresses to the same physical interface. Classic: eth0:0, eth0:1 (ifconfig syntax). Modern Linux: ip addr add 10.0.0.5/24 dev eth0 — multiple IPs on the same eth0 without alias suffix.
Use cases: multiple TLS-cert IPs on the same server (SNI has eliminated the need), virtual IPs for VRRP failover, separating services on the same host to different IPs for access control. Loopback: lo has default 127.0.0.1; add 127.0.0.2 for local tests of multi-host setups without multiple machines. Cloud: AWS EC2 secondary IPs, GCP alias IP ranges — talk about the same concept but hypervisor-managed. Kubernetes uses secondary IPs heavily in many CNI implementations.