The term ‘C-net’ is commonly (but often erroneously) used to describe an Internet Protocol (IP) Network with the netmask 255.255.255.0, meaning it contains 256 IP-addresses (.0 - .255). In most cases, the .0 and the .255 are reserved for network specific functions, leaving 254 functional IP-addresses.
Since the introduction of Classless Inter-Domain Routing (CIDR) in 1993, that netmask now corresponds to a ‘/24’ (eg: 192.0.2.0/24, meaning it contains all the IP-addresses between 192.0.2.0 and 192.0.2.255). In the predecessor to CIDR, Classful Networking, the class with netmask ‘255.255.255.0’ was ‘1 C’, which is why the terms became erroneously synonymous.
The tool above will check the reverse DNS (the hostname associated with an IP) for all IPs within the entire network (in this case within the /24). Simply enter the first three segments of the IP-address (eg: a.b.c, or 192.0.2) and the tool will do the rest!