A few months ago, I found myself wondering how a command like ping 1.1.1.1 works from within a private network. In most private networks, multiple hosts connect to the Internet through a router. For IPv4, the router performs network address translation (NAT) by rewriting the original host’s source address to the router’s public IP address. The router can lookup the correct host for a reply packet based on the packet’s port field, at least for protocols like TCP and UDP.