Port Filtering By ISPs
Many Internet service providers restrict their customers' ability to perform port scans to destinations outside of their home networks. This is usually covered in the terms of service or acceptable use policy to which the customer must agree. Some ISPs implement packet filters or transparent proxies that prevent outgoing service requests to certain ports. For example, if an ISP provides a transparent HTTP proxy on port 80, port scans of any address will appear to have port 80 open, regardless of target host's actual configuration.
Read more about this topic: Port Scanner
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