High speed Internet connections that connect through a cable service provider use a cable modem to access the Internet. Unlike dial-up Internet connections, cable connections do not need to "dial in" to connect and they also seem to be "always on"--as in, connected all of the time. A modem flap is the number of times that the connection has been dropped.
A flap is measured in numbers. The number given when an IT or cable representative runs a diagnostic test on a modem is the number of times that the modem has gone offline and then come back online since the last time it was turned on.
High flap numbers may indicate a connection problem. Connection problems can range from hardware issues with the modem itself or due to high periods of traffic on the cable provider's connection.
Flap is also significant because cable Internet is often advertised as being "always on" when in reality flap information can show thousands of drop-offs and reconnects. An example can be viewed in the References section of this article.
The more instances of the modem being knocked offline naturally results in more instances of the modem attempting to reconnect. Flap numbers are often in the thousands but most of the time these offline moments are so brief that they go unnoticed. However if a modem's flap is extremely high (in the tens of thousands), these disconnects can become noticeable.
The number shown for a modem's flap does not indicate the exact times that a disconnect occurred. As for the length of a disconnect, it can vary from a fraction of a second to hours. In short disconnects the modem usually attempts to reconnect as quickly as possible. However sometimes the user has to disconnect the modem power and let the modem reset before a connection can be reestablished.
Sometimes the flap number can be lowered by replacing the cable modem itself. Other times the issue lies with the cable service itself or the amount of traffic currently on a cable service provider's network. It is important to note that cable internet connections are often split among multiple subscribers; periods of heavy use may increase flap.
Each modem and cable service provider may use a different system to run diagnostics. Contacting the service provider may be needed to determine exact flap and other statistics.