RFC_1918 is a document that defines a set of private IP address ranges that can be used within local networks. These addresses are not routable on the public internet, meaning they are meant for internal use only. This helps organizations avoid conflicts and conserve the limited number of available public IP addresses. The three main ranges specified in RFC_1918 are 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255, and 192.168.0.0 to 192.168.255.255. By using these private addresses, devices within a network can communicate with each other without needing a unique public IP address for each device.