The next tool I will be reviewing is from the same suite of tools as netenum and netmask. Protos is a IP protocol scanner. It goes through all possible IP protocols and uses a negative scan to sort out unsupported protocols which should be reported by the target using ICMP protocol unreachable messages.
Netmask is another simple tool which does one thing and that is, makes a ICMP netmask request. By determining the netmasks of various computers on a network, you can better map your subnet structure and infer trust relationships.