Doing nmap on a network

Question

I used nmap to scan all the ports in the hosts in a network by using the command:

$ nmap 172.31.100.0/24

What I found is that it showed the following in the result:

Nmap scan report for 172.31.100.0
Host is up (0.0039s latency).
Not shown: 998 closed ports
PORT STATE SERVICE
23/tcp open telnet
80/tcp filtered http

What does 172.31.100.0 represent? Is it a specific interface or something else?

Answer 1

172.31.100.0 is the IP address of one of the hosts you scanned. If your network is actually 172.31.96.0/21 (or larger), then 100.0 is a perfectly valid IP address.

172.31.100.0 is part of the pre-CIDR Class B IP space, so you may have gotten a default network of 173.31.0.0/16 if you didn't configure otherwise (and 100.0 completely valid on that network).

If you don't want to scan .0, you can invoke nmap with a range: nmap -sS 172.31.100.1-254.