How to get access to the IIS Websites using Host Name without configuring the host files

Im currently want to setup websites using the IIS. I'm using the AWS Server. i have 2 websites which i want to setup which is www.a.com and www.b.com.

for the first websites www.a.com, i create the sites name websitesa. i already done doing the Bindings to this websites which is

1. www.a.com with port 80 and IP address *
2. www.a.com with port 8000 and IP address *
3. no hostname with port 8000 and IP address *

for the seconds websites www.b.com, i create the sites name websitesb. i already done doing the Bindings to this websites which is

1. www.b.com with port 80 and IP address *
2. www.b.com with port 8000 and IP address *
3. no hostname with port 8001 and IP address *

i also done doing add the information into host file. www.a.com www.b.com

then i am able to access to websites using:-

1. my local pc , i browse to the www.a.com or www.b.com then it will redirect me to the correct websites.
2. using other pc, i unable to access to the www.a.com or www.b.com. then after research, i add the same information into host file in the other pc. then after that only able to access.
3. using my phone, I'm unable to access to the www.a.com or www.b.com as i cannot modify the host file of the mobile.

my concern now is, is it i able to use the websites without any hosts file configuration? is there any missing steps? because i want user to be able to access to the websites without the need of us to do the host file configuration. it would be tedious as if let say got 1000 pc/devices then i need to configure hosts file for all the devices.