RFC1918 space is the set of private IPv4 address ranges reserved for local networks: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These addresses are not supposed to be routed directly on the public internet. Instead, they are used behind NAT, inside homes, offices, VPNs, and lab environments.
In cyber security, RFC1918 space matters because it defines what should stay internal. If a proxy, relay, or misconfigured service can reach private ranges, it may expose internal-only systems such as admin panels, file shares, or APIs. Attackers also scan for RFC1918 targets after gaining a foothold, because those addresses often reveal trusted infrastructure. Defenders use segmentation, routing controls, and access policies to ensure that private space remains private, even when endpoints provide internet access or relay traffic.