ipcalc - IP Calculator. ipcalc takes an IP address and netmask and calculates the resulting broadcast, network, Cisco wildcard mask, and host range. By giving a second netmask, you can design subnets and supernets....
whatmask - Whatmask is a small C program that will help you with network settings and subnet calculations.
Subnetter - Split IPv4 network ranges
Port Authority - A handy systems and security-focused tool, Port Authority is a very fast Android port scanner.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.