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
Famous quotes containing the word port:
“The triumphs of peace have been in some proximity to war. Whilst the hand was still familiar with the sword-hilt, whilst the habits of the camp were still visible in the port and complexion of the gentleman, his intellectual power culminated; the compression and tension of these stern conditions is a training for the finest and softest arts, and can rarely be compensated in tranquil times, except by some analogous vigor drawn from occupations as hardy as war.”
—Ralph Waldo Emerson (1803–1882)