Can't access localhost from the internet but works fine on "localhost"?

[Silvercat]

I have a laptop with win 8. I can access localhost locally but when I type the public IP address then it doesn't work. I tried changing Apache to port 81 and still the same result.

I use a HSDPA modem with a SIM card to connect to the online. No router nor this is connected to any LAN

[Uday]

I think there's some problem with your host file. Please try to give a look to it.

[me2]

Your question is a bit ambiguous.  "LOCALHOST" is the computer that you're using.  The address of LOCALHOST is 127.0.0.1, which is a loopback address not routed to the network, and therefore cannot be accessed externally.

If you mean that you cannot access another computer by typing its public IP address, that is normal if the computer isn't running a server with which to respond.  Although you mention "no router", the modem may in fact do network address translation (NAT), and the computer which you're trying to access may connect through a NAT-capable device, either of which would normally prevent the connection.

[roderick_young]

Is there any firewall on your system? That might be blocking port 80 from getting in from the internet. If it's Windows Firewall, create an exception for port 80. Same thing if you have zonealarm or any similar protection software.

It could also be that your ISP is preventing you from using port 80 incoming. To check that, take your laptop to a friend's house, and try it there, hardwired to the internet, and with port 80 forwarded through the router.