Networking

For checking connection states, confirming what is listening, and counting per-protocol connections, netstat is the veteran Linux command, complementary to ss. This reference covers listing connections, filtering by TCP/UDP, listening ports, the routing table, and interface stats, and flags -p to pin down the owning process.

netcat (`nc`) is a Swiss-army knife for network checks: test whether a port responds, stand up a throwaway listener, relay between sockets, or move a file — without installing a server. This reference covers connect/scan, listen-and-echo, HTTP debugging, forwarding, and UDP mode, for quick connectivity checks under pressure.

CIDR notation reference covering subnet-mask tables, common subnet sizes, private IP ranges, special-purpose addresses and the math to split or merge subnets. For network planning, ACLs and diagnosing addressing mistakes.