To find all IP on a network with Port 80 Open in a network, run

nmap IP_START-IP_END -p80 --open

Example

boby@hon-pc-01:~$ nmap 100.71.116.1-254 -p80 --open

Starting Nmap 7.01 ( https://nmap.org ) at 2017-07-30 21:26 IST
Nmap scan report for 100.71.116.60
Host is up (0.0045s latency).
PORT   STATE SERVICE
80/tcp open  http

Nmap scan report for 100.71.116.77
Host is up (0.025s latency).
PORT   STATE SERVICE
80/tcp open  http

Nmap scan report for 100.71.116.88
Host is up (0.014s latency).
PORT   STATE SERVICE
80/tcp open  http

Nmap scan report for 100.71.116.146
Host is up (0.016s latency).
PORT   STATE SERVICE
80/tcp open  http

Nmap scan report for 100.71.116.154
Host is up (0.025s latency).
PORT   STATE SERVICE
80/tcp open  http

Nmap scan report for 100.71.116.218
Host is up (0.012s latency).
PORT   STATE SERVICE
80/tcp open  http

Nmap done: 254 IP addresses (91 hosts up) scanned in 6.77 seconds
boby@hon-pc-01:~$ 

• nmap